Make up your mind, are they just to titillate, or are they to inform us about the characters? One I find justifiable, the other I don't.
titillation and sex has to inform the characters. I don't see where you're inserting a disconnect. name a film where we're shown a wholly nonromantic couple and then we cut to them banging and then cut back and their having sex has absolutely 0.00 effect on their relationship from that point forward. Like, a film can definitely have sex scenes that aren't justified but a film can also have so many other kinds of scenes that aren't justified. events that are otherwise incongruous with characters. events that attempt to provide depth of insight into a character that fail to do so. that you're singling out sex in particular is peculiar. if all you meant was "I don't like when an event in a film isn't justified and when it pulls me out of a film," why not just say that? the way you put it was that sex scenes specifically are bad because they often titillate and nothing more. I'd argue that every kind of scene in every movie has a chance of being empty. most movies are not good. so is your argument simply that sex scenes (or scenes of violence) have a higher probability of being unmotivated? even then I don't get why that argument is vital, but at least it kind of makes sense
edit: oh right and I meant to add, sometimes the only justification needed is entertainment. dont' see why that can't be sufficient justification when done correctly.