I've been in plenty of long-term relationships and one of the several reasons things have not worked out between me and my exes are our views on sex and our sexual relationship. The actual act of sex never suffered though and it was always fun because I enjoy being with my partner in that way. It's not simply about getting off.
As far as I'm concerned, if I am in a relationship for the long haul, then I am immersed in it. Maintaining a relationship is full-time work and you don't get sick days. There are off days, sure, but it's important for the health of the relationship and the two people involved that both parties remain invested. Sadly, one or both parties tends to quit early, and a lot of those people fail to own up to their issues proactively. A lot of people wait it out until the frustration and resentment have already set in. The sex becomes bad or lackluster, you fight about stupid things, have numerous conversations about "fixing things" to no avail. Those are all signs of deeper issues with those individual people. And instead of talking about it, each person is afraid that the other will just call the whole thing off. So things go stale and remain stagnant and those people become miserable humans because of it. They stay together out of fear and to avoid being lonely.
Sex doesn't have to become formulaic. It doesn't have to become a transaction where one person gets to have an orgasm because they took out the trash that day. For me, when I have sex it is about sharing the bond I have with my partner and it is about making them happy. Sure, I'd love to have an orgasm every time, but it doesn't always happen like that, and that's okay. I want my guy to want me because of who I am, not what I can do for him.