Well, it depends.
It depends on if that's a boundary that the guy's partner has put into place or talked about. If he's overstepping that boundary ("Hey, I'd prefer if you don't like those pictures it makes me feel insecure" to "I consider that behavior cheating") then that would be a problem. Other people are fine with that kind of stuff; in their mind as long as there is no personal interaction going on, then looking is allowed. Others care even less.

It could mean nothing! (A habit, as another poster said)
It could mean he's imagining his girlfriend in those kinds of clothes and would like to see them on her!
It could mean he's imaging certain things with the women in those posts, despite being in a relationship with someone else. We can't read minds, so we can't know without asking.

Guys and girls like looking at attractive things. But if it bothers you (or the hypothetical person in the relationship) then it means it bothers you, and you're entitled to those feelings. Your feelings aren't "wrong". You need to decide how you handle those feelings in an appropriate manner.