So here is the scoop (we are a gay couple, by the way):

When we first met, I lived in the city, and he lived in the suburbs. He would drive to see me on the weekends, every single weekend, without fail, for the near first year we dated. Our sex life was pretty darn good, usually having sex at least once, but sometimes twice and even three times during those weekends. Finally, after nearly a full year, we moved into an apartment in the suburbs. I gave up a very close commute to work and my apartment to live with him.

Right at first, we still continued to have regular sex, and some amazing experiences, but shortly before he returned to school to get a new degree (that keeps him very busy), our sex life dwindled away to nearly nothing. I also noticed that he seemed to have sex to make me happy rather than from any real need or desire. Evidence of this included getting soft when beginning, a lack of reciprocal touch, and the change in the way he would approach me.
At this point it's been a little over two months since the last time we had penetrative sex. When I allude to sex, he says no. When I ask, he says no. When I try to seduce him, he calls me weird or fails to respond. His general excuses include: something he has to watch on TV, he has homework to do, he's hungry, he's not the mood, he's tired, he has a headache, or I'm ruining the mood.

I've gotten to a point where my self-esteem has hit rock bottom. I can't help but think something must be wrong with me despite knowing I'm a good looking man. I am out of options and tricks to get him to remain intimate with me. When I've tried to discuss it directly with him, he says that we're fine and that sex isn't that important (despite our sex life for our first year), or he says "we just had sex"... so either he's remembering sex with someone else or he's really that deluded.

I keep thinking that he's cheating on me with SOMEONE, but I can't see how. I know where he is at all times and there is enough evidence that he isn't to make me fairly sure he's not, but I can't be positive.

I'm trying not to end everything, but I can't think it'll get better.

I always seem to be posting the same answer - you are in a relationship. You must talk about other topics that trouble either one (or both) of you.

So ask HIM if there's a problem, if he's "cheating," why he is not interested in sex. Maybe it's a school concern, maybe a health problem. Presumably he's the one who knows.

I'm an investigator - I have found throughout the years that if someone THINKS someone is cheating because there's a change in behavior it's either a health issue OR the person is cheating.

Ask him - don't be confrontational. Just ask for your own peace of mind. If there's a shred of the relationship left, he'll answer truthfully.

Or are you done with the relationship?

First he is correct in that a relationship is not based on sex, unless that is all it is. If it is just great sex to you and not a emotional relationship, then it is ending for you.

He may have a ED that he does not feel good talking about.
He may have stress from school and is too tired to think about sex as much.

And it is not looks, that is just physcial and again, if looks is or was all that was important, those relationships don't last long.

You and he need to talk, really talk

Why would you presume anything before you talk about it?

It could well be that other areas of the relationship need work, and those issues can effect the sex life.

When the lust has waned there has to be other things that keep a relationship hot.

Talk about it.