Having had both Son's and YFs I can say what its been like for me.

I love my son's and my yfs differently.
There is a huge difference between paternal love and romantic love.

The love that is in common is the friendship love. I feel that with both. Also it's possible to feel paternal towards my yfs from time to time... but generally it's friendship/romantic love I feel.

The commitment is different too. A son is a life long commitment and responsibility that starts from birth. It's more devoted really than a yf situation allows. You have to provide a level of care for a son that you don't usually for a yf. You have to change your kids' nappies, take them to the doctors, work to provide food for them... it's a different kind of love giving to what we do for a yf and it's rewarding in a different way.

My son's give me enormous joy and delight... but they can't make my head swoon and spin with joy the way my yf can. One could argue that I love my sons in practical ways more than my yf... but I love my yf more in an affectionate way than I do my sons. It's different yet I would say... somewhat equal; I feel I wouldn't want to live without ANY of them.... and I'd give my life for ALL of them.

Of course you may be in a situation where you do provide paternal care for a yf... in which case he does become like a son to you.... I have one yf that was like this.... on the other hand you may be attracted to your sons... though I don't think I could ever be "in love" with them in a romantic way.

These things are never black and white.
Blessings
Cat.