Guys: is the main point of a relationship sometimes just sex?

What the title says really. Understandably most guys aren't going to want to go out with girls they don't find attractive, but at the same time do you ever start relationships just/mostly for the sex, not a girl's personality and how much you like her as a friend? Obviously there are options like one night stands or friends with benefits but those are temporary or harder to get a girl to agree to. Me and my best friends have a few close guy friends from home who've been in relationships for a couple of years and often turn to the girls in our friend group for advice because they aren't sure if their girlfriends are "the one"/they don't really do much together any more or feel particularly compatible as they go to different unis and they've changed since they got together. One of them even says he fancies a girl at uni he's known since first year and gets along really well with, and he'd "sleep with her over" his current gf "any day of the week", but he also thinks she's out of his league and might reject him so he won't break up with his current gf even though he clearly no longer respects her or finds her attractive. Nevertheless they stay in these relationships just because, they admit, they're getting regular sex. We know some of those girls as well, they're all convinced their relationships are going fine and their boyfriends are happy, but we haven't told them anything although it's a tough issue b/c we don't want them to just be strung along and used for sex. So, guys, do you ever start/continue relationships just or mostly because you can get sex? We understand this isn't always the case... but we're curious as to whether it's the case more often than girls think. :)

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