[...]but it would be nice to be able to avoid sexual / intimate interactions with the guy's, without having to be a bi... no better not say that... without being mean, just telling them that you are a Lesbian, or that you are in a committed relationship or something.
You are not wrong. Having one choice Alice makes right at the start be the deciding factor about a sexual scene with Marco way later without any weight being given to posterior dialogue choices is bad design.
...or just being able to straight-up say 'no' to sex/intimacy with a character the player doesn't want. You'd think that would be the most obvious choice, considering the realism and simplicity of such an option, right? I'm not sure why there aren't more multi-romance player-choice games that offer such a simple thing, especially in games that are designed to validate the player's choices.
I mean, it may be kind of jarring in the situation, but it's not as if you'd care about that if you don't want to be in the situation in the first place.
I think it's a must for the game to adhere to a choice the player makes about their sexuality. However, I think it's probably better and more realistic to give the player the choice of what to say/how to act towards a character in a given situation, and give them the choice to reject or reciprocate any advances a character makes toward them. Agency should always remain in the players hands in choice-based games, and choosing sexuality in a game can actually achieve just the opposite.
So for example, when you've stated you're a lesbian, the game may then assume you want to flirt with and/or are interested in every female in the game. For some people that's just fine, but for other who have more specific taste it can be really off-putting. I think for
most people if the game takes the initiative for your character and assumes their actions, your character can end up acting totally different from what you would do if you were actually given the choice in that moment. That's a massive no-no in a game where you're supposed to be able to play the character as you envision them.
Personally, I'd prefer to be able to choose who I'm interested in as I meet and get to know them. As for the other character's actions towards your character, just like in reality it makes sense that you're gonna have people you're not attracted to at all try it on with you, so let that be the case, as long as you always have the choice to reject them.