It depends on what you mean by "draw the line". If it's "I will not play it if has that content", that would be rape.
And since "rape" has varying definitions, with some people unhelpfully trying to put every type of non-con under "rape", my definition is this:
1. Sexual contact without the victim's consent.
2. Sexual contact in which the victim is physically unable to give consent (e.g. passed-out, sleeping, gagged and bound)
And while other types of non-con aren't deal-breakers, I prefer to avoid them where possible, with coercion, blackmail and financial exploitation being the worst. I dislike seeing them in my favorite fetishes (corruption, NTR). Some of the others mentioned (drunkenness, deception) are more palatable, though they should still only be used to get the ball rolling. Peer pressure I feel is an ordinary part of social life and doesn't really qualify as any kind of non-con as long as the person is free to disengage from their peers.
Looking at some of my favorite games, let's see what level of non-con they have:
Overwhored: Invasive hypnosis against the victims' will. Essentially rape as it takes away their ability to consent (though it's pure fantasy stuff), even though they enjoy it. However, that aspect, plus the mental enslavement of heroic characters, are what actually makes the non-con appealing for me in that game.
iNSight of you: Mind-control, coercion and blackmail. Extremely non-con overall, though the victim is such a terrible person that it's hard to feel too sorry for her, and I believe this is intentional. Actually, spite toward the character just adds to the appeal of seeing her humiliated in every way. Not to mention she's wired to get off on that treatment.
Karryn's Prison: Everything besides stripping and restraining Karryn requires her consent. They can't even cum on her unless her desire for it is high enough. This I like, especially since you can decide just how far she's willing to go.
News Desk:
Henteria Chronicles Ch. 2:
So yeah, a lot of my favorites have non-con aspects, but all of these have some way of softening the blow, so to speak, usually with fantasy or sci-fi elements. Games like this are far outnumbered by legions of games that are too rapey (like 90% of what comes out of Japan) or too vanilla (insert almost any popular VN here). Personally, I know I prefer corruption / moral degeneration arcs that are based on people's inherent lust, pride, greed and selfishness. Non-con often cheapens that theme, though I realize some characters need prodding to get things moving in a believable way.