I've read some studies (not at hand so I can't cite them), that talk about the human male response to mating. Now this is straight sex stuff, not gay, but I've always believed that gay & straight men are more alike in their behavioral patterns than we gays might care to admit.
And the studies suggest that when a human male mates with a female, he becomes predisposed to emotional attachment with her. And this would make sense in an evolutionary context, since a pregnant and later nursing mother is more likely to be successful in bringing a child to the point of self-sufficiency, when an adult male assists her during this difficult period.
[A corollary to this theory proposes that the so-called "seven-year itch" occurs because the mother & offspring are now sufficiently independent at that point to allow the father to move on to start a new family with other females. Why he would not remain and continue to reimpregnate the same female is not explained.]
Therefore, having sex with someone may trigger automatic emotional attachments in human males. Attachments that manifest themselves as fondness & dedication, and the tendency to overlook & minimize logical objections & obstacles to a long-term union.
To put it in blunter terms: when you put it in, you may find yourself getting stuck in ways you hadn't anticipated. And that's the way Nature intended it.
The knowledge of what is happening here may help you to resist someone you've already told us is not a good candidate for an LTR with you.