That's the thing, you just said it's an open question whether Ian would resist Gillian, but it's not an open question in most cases for Lena and that makes it not the exact same situation.
OK, i guess this is mostly arguing semantics like that. I should've perhaps worded it better/more precisely than "the exact same situation". So i'll concede, this isn't 100% exact same situation in all of its aspects
But regarding your theoretical example, i honestly can't say if my personal reaction to it wouldn't be more of a "well, maybe" than "hell no" which is why i think this could influence Ian's view, and since you view it differently, we have this disagreement.
You have a point on inconsistency but I would argue for the opposite way. Ian should not be able to enter in any serious loving relationship if he's not over his ex. But that's just my opinion, it would probably be best to allow for player choice to appease different opinions on the matter. But I guess it would also mean more variations than we already have and make writing to account for all of them even more complex than it already is.
I think it'd be very constricting to have requirement like this, and also far from realistic. People most definitely don't run this sort of mental calculations of "am i fully over my ex or not" when they decide to get in new relationship. Heck, you'll find people who go as far as full marriage while still having hangups. There's no such thing as "you can only really love one person" and the game itself even acknowledges it with the throuple route.
Of course, such decision-making doesn't have to end well (and often doesn't) but that's half of the fun in a "choose your relationship mess" porn game, imo.