Mt 19:

"Haven't you read," he replied, "that at the beginning the Creator ‘made them male and female,' 5 and said, ‘For this reason a man will leave his father and mother and be united to his wife, and the two will become one flesh'? 6 So they are no longer two, but one flesh. Therefore what God has joined together, let no one separate."

7 "Why then," they asked, "did Moses command that a man give his wife a certificate of divorce and send her away?"

8 Jesus replied, "Moses permitted you to divorce your wives because your hearts were hard. But it was not this way from the beginning. 9 I tell you that anyone who divorces his wife, except for sexual immorality, and marries another woman commits adultery."

So here Jesus clearly endorses the creation story as the basis for morality. In that, the solution to lack of relationship is provided by a single other person (let's stay away from the separate 'gay v straight' debate for now'). The two 'become one flesh', and this is the basis for his rejection of divorce and remarriage.

Paul extends the 'one flesh' concept to include the effect of having intercourse with a prostitute:

'Do you not know that he who unites himself with a prostitute is one with her in body? For it is said, "The two will become one flesh." (I Cor 6:16).

So if even casual sex creates 'one flesh', how much more an extended relationship, however much 'love' is or isn't present.

Note then that writing in this context Paul continues: (v18-20)

Flee from sexual immorality. All other sins a person commits are outside the body, but whoever sins sexually, sins against their own body. 19 Do you not know that your bodies are temples of the Holy Spirit, who is in you, whom you have received from God? You are not your own; 20 you were bought at a price. Therefore honor God with your bodies.

and previously had commented in v.9-10

"Do not be deceived: Neither the sexually immoral nor idolaters nor adulterers nor men who have sex with men nor thieves nor the greedy nor drunkards nor slanderers nor swindlers will inherit the kingdom of God."

This passage is usually thrown around in the question of what is meant by the reference to gay sex; what however is far less ambiguous is the warning about sexual immorality. AFAICS, the only way to interpret this collection of passages is the traditional Christian understanding: sex should be restricted to covenanted relationships that are intended to be permanent and nowhere else. I would tend to add that these should only be between a man and woman - but you can finesse that without undermining the strength of this logic.

And we need to hear the degree to which Paul is making a big deal of this whole area:

"All other sins a person commits are outside the body, but whoever sins sexually, sins against their own body. Do you not know that your bodies are temples of the Holy Spirit"

Sexual sins really ARE worse than others in some sense; Paul is writing to a church in a city where sexual immorality appears to have been a very strong feature of the culture - and he calls for Christians to live radically differently. We are in a society where sexual immorality is a strong feature of our culture: we should surely do likewise.

Critiquing this position requires you to offer an alternative definition of 'sexual immorality' that makes sense of this usage. I've yet to encounter one. We therefore need to try to live by what God requires, which surely means celibacy outside marriage and faithfulness within.