The second time this has come up today. Like you said, a point worthy of much discussion. I'm glad you brought it up.
I am fundamentally with you on this matter. Let me give you some background as I know it: a good VNs is to me a work of art and a piece of brilliant literature. As far as I am concerned, there are a million and one other, BETTER ways of illustrating romance and/or helping your readers "get to know" your characters ASIDE from letting them take them to bed. Unfortunately as you have said, sex has indeed been a major part of the medium and it shows little sign of ceasing to be so (I could write long essays on why I think this to be the case but unless anyone is interested I won't).
As such my stance is this: If it does have sex, it is something that is at best "there just cos' it has to be" or; at worst, filthy mud that taints the brilliance of what is otherwise a radiant star. Seriously though, sometimes even if the scene is perfectly in context the very thinking behind their undertones just makes me wanna scream! Freshest in my mind as an example is Daibanchou. Basically, you're a banchou (sorta like a gang leader) that fights other people with the ultimate aim of destroying all evil. At regular intervals you are offered a "heroine sex phase" where you can select your lover for the time - with the effect of giving you bonuses like extra stamina or more money. Having sex with girls to get more money. I mean REALLY NOW. I don’t even need to START saying in how many ways that’s WRONG.
Anyways practically speaking, it depends on what you are comfortable with. A proportion are not comfortable with seeing sex full stop - in which case I direct them to the many great VNs that don't have any. For the rest who can tolerate it up to a point, I encourage extensive use the “skip” button. (you can hold down “Ctrl” on the keyboard to speed through text in almost all VNs).
The other thing is, you should be able to tell from the cover and title if there is excessive sex is in the game. I shouldn’t need to give examples - though I include with this advice the notable exception of “Monster Girl Quest”. This game follows an RPG format and came highly recommended by many. I began playing it and thoroughly enjoyed its gameplay, storyline, and satires on standard RPG cliches.
...then I discovered the full title was “Monster Girl Quest ~ Lose and you will be raped by monster girls!” So I thought: ok fine, then I'll just keep on making sure that I don't lose!
...that’s when I discovered you can’t get a 100% completion rate for the game's "monsterpedia" unless you've made yourself lose to every single enemy in the game.
... -_____________-
I continue to cultivate my propensity for tolerance as I wait with bated breath for the release of Monster Girl Quest 3. Watch this space.