Man loves boy, and welcomes thoughts of sex with boy.

Boy likes man, quite possibly remains still unaware of the full extent of the parameters of his sexuality. Is the boy gonna be gay, or bi-sexual, or straight?

Man loves boy, with or without sex, but favoring ideas of a future including reciprocity of love with sex.

Let's say the boy loves the man, but the boy is not gay.
Perhaps the boy might want to experiment with sex, result of the loving friendship, but deep down inside, if he's heterosexual, no matter how pleasant the sex might feel, will not motivate further deepening passion as it would do if he were totally homosexual.

Now then, man loves boy, but man feels hurt that boy cannot extract sufficient meaningfulness from the gay sex as he can.
Man feels it might be best to try and not love the boy so much then, because this is painful, loving the boy, sharing friendship, but his view of the sex is not so amazing and enjoyable, but instead rather probably kinda gross.
boys like gross stuff, so some will do it anyways!

Now, if the man can love the boy enough to share a moment of sex, and handle the inevitable rejection of any serious future advances, and remain friends with the boy, they can put their short sexual excursion out of mind, forget about it. Just boys have fun together as buddies.
This is what Eldad means saying you must love the child enough to respect his lifestyle choice, you have no right feeling butthurt just because boy cannot grasp how awesome being gay is.

Man who feels hurt because boy is not gay, really does not love the boy.

further on, let's say the boy is gay, and the sex is good, and the friendship is good. thus the man is delighted, gearing to deepen this perfect found relationship. but, young boys are still only learning. maybe the sex solidifies his discovering his true identity as a gay person, so he entertains other options, other men or boys his age, for love or sex. is it okay for the man to be hurt and pissed?
Again, you have to love the child enough to be able to handle the worst possible scenario.

next, to answer your other questions you raised regarding what Christianity is and isn't.

If you and the boy love each other, and sex happens between you, simultaneously to what happens in bed, you two are come together before the altar of the Lord, with God and the Angels all floating around above you watching you two having sex in your love. A little wedding ceremony happening in the spiritual realm, in heaven.
You do not need a justice of the peace or ship captain or a reverend to marry you. Marriage is not a piece of paper. Your marriage has absolutely nothing at all to do with the preacher guy instructing when to say "I do vow" the ceremonial rituals for marriage do not marry you, it is your love and sex that marries you together in Gods eyes.

Just because your state doesn't have gay marriage, doesn't mean gays aren't married there in God's eyes.

Having sex with a boy, or anyone, means forging a bond that is eternal.
Even if you both forget having sex with each other, God doesn't forget, he remembers and knows the truth of what happened.

Next you ask how homosexuality is sinful in christianity?
Homosexuality is not sinful to christianity. Let me explain to you what is chastity and what is premarital sex, and what is holy or sinful.

It is horrible some of the kooky beliefs false christian organizations project through the ages. like catholics who oppose masturbation.

The truth is, every normal wholesome person, boy or girl, masturbates on an average typically about once per day. that is normal and healthy and christian. it is not unlike taking a shit or a piss, it occupies the boredom, get's the heartbeat going, provides a tiny dose of endorphins.

It must be understood that God loves sex and wants us to have it, daily.
Sex is beneficial to body and mind, and only becomes dangerous or problematic when it is dysfunctional, warped, unwholesome, tainted by lack of love.

So, when it comes to having sex with a boy, and determining whether or not the action is sinful or wholesome, well, it depends on how good the sex is, how appropriately honestly executed.
For example, whether or not you are aware of the child masturbating once per day at home, if you are in the home while he is doing that, well, you're sort of having sex at that point, insofar as you are aware of one another.

There is nothing sinful about people in a family, friends of family, masturbating secretly in private under the same roof. That has been happening since the dawn of time and will always be so.

Let's say boylover walks into a room discovering a boy masturbating.
Is it any different than if his mother accidently walked in?
She might be shocked or repulsed to turn away quickly and forget about it.
The boylover instead feels excitement and satisfaction. Wishing to stay in the room, but not to disturb or directly engage the boy.
Are they having sex at that point?

See, sometimes exercising sexuality is wholesome, and sometimes not.
Hardcore anal marathon sex is likely inappropriate with most average boy children. The idea is sinful. A wholesome person is not going to deem it necessary to display his love and affection thru sodomy.

You know, laying alongside of a child who is masturbating, under a blanket, minding his own buisness, somewhat secretly halfway privately, that is not even illegal. is that considered having sex?
maybe, but i can tell you it's not considered sinful.

Let's say later on that night, after your nightly prayers, laying down in bed you recollect the memory of how exciting it felt hearing the boy breathing heavy enjoying himself thru his boredom. so you masturbate fondly over the memory. Now, you're not having sex together with the boy, but separately under the same roof, close enough, you basically both almost nearly jacked off together. You had sex in God's eyes, in a very minimal sort of way. And that is not sinful.

Maybe a fine line between sometimes. For example, let's say after spying the boy fapping beneath his blanket, you're motivated to go and drum up some hardcore child porn to watch, searching for a boy who looks exactly like your beloved, so you can indulge in filthy thoughts of sodomy. now this is sinful because you're tainting something that was innocent and wholesome, making it something dishonest and offensive, to be hidden.

If you need help determining what is sinful or not sinful, think of it like this. If you are magnifying the glory of God, by way of love, then it is not sinful. You must ask yourself "...to what end?"

You'd like to consider future sex with the boy? ask yourself, "...to what end"
Do you want to improve his life, mindful of his personal welfare?

To what end is your desire to sex with the boy?
For your selfish orgasm, or for his? or is it to educate him? to nurture his self-discovery.

nurturing his education and self-discovery to what end?
To magnify the glory of the lord? or to manipulate a future with him the way you want it, prioritizing over his needs?

My personal opinion, age 16 he's a grown man, and there is nothing you could do to hurt him. unless he's a complete total pansy.
I think it's time the boy realize that yeah, some men think he's hot and want to get with him. get used to it bucko.