I've heard that having an orgasm is synonymous with being under the influence of heroin, the way the dopamine kicks in & all that other stuff. It has the potential to cause a very relaxing effect on the body.
Perhaps the body feels it has done a lot of "work" to achieve orgasm, you know, get the blood pumping & what not, that it feels exhausted afterwards.
I have been guilty of starting to doze off while receiving oral sex. But consider the conditions. Nice hotel bed. Air conditioning. Late at night after a long day. All of the factors that contribute to making one relaxed, and also tired.
So in your case, it can be any of a combination of what the sexual activity is doing to your body, or the conditions in which you are having it, like the when (what time of day) & where.
I've actually dozed off while jacking off, & frequently nod off after jacking off. Although we're talking in the overnight when i haven't gotten much sleep, and maybe i'm also under the influence of something which could make me sleepy too.
You may want to try first thing in the morning after waking up. Or go out & get a nice coffee or an energy drink or something first to help stay awake & alert.
It's ironic that you feel guilty for being unconscious while someone is having sex with you. One would think the one who should feel guilty is the one having sex with an unconscious person. But i get it.