Why would sex just stop in a relationship?

I’ve been in a committed relationship with my partner for about a year and a half. We met in a hotel lobby and from day 1 the sex was amazing. It was a big part of what attracted me to him to begin with. That is until about three or four months into the relationship it slowed down significantly and then came to a screeching halt. I’ve had more than one argument with him about it and he insists that it’s nothing I’ve done and he’s still attracted to me. For the longest time I thought he may be cheating but now I don’t think so. He has no physical/medical condition that would prevent him from performing. I just can’t get an answer out of him and it’s driving me nuts. I’m only 31 and he’s 25. He should be like a wild animal. I know I am. I will not cheat on him because I love him so much but I have needs. Other than the sex issue our relationship is pretty much perfect.
Sex is no big deal until you’re not getting any, then it’s a REALLY big deal. It’s been three months. Need advice!