I would say that you're both right and wrong.
I would never dream of pushing someone into being sexual in a way that they didn't want to be, and have plenty of friend relationships that aren't sexual. Sometimes, I'd love it if they were, but see the start of the paragraph... I'd much rather have them as friends.
But on the other, I have been in a sexual relationship where the sex stopped and it was the beginning of the end. Plus some sorts of sex are seen as deal breakers by plenty of people, especially around (not) having kids.
Here, if A wants B to go down on them but refuses to reciprocate, then it's up to B whether or not to continue the sexual relationship, not us. If they do there are, again, several options including getting it via a relationship with someone else as well. That could be done in a variety of ways too - there's something I enjoy a lot that my partner doesn't, and I get to do it (but not some other stuff) with others.
It's a matter of negotiation, one way or another, and not every deal can be made.