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No problem! I like to play early versions of games to give such feedback ^^

On this point, I am of the mind that if not every sex scene, at least the first time with each major heroine should be preceded by a choice that unequivocally communicates to the player that the result will be a sexual act. 

I know the Eternum comparisons must be getting tiresome to hear, but imo the golden standard of such choices is the choice given there before the first Annie scene, when her and MC sleep topless together. The entire mood of the scene, plus the thoughts of the MC that come right before, make it so the choice can't be understood in any other way than "do lewd stuff/don't", giving players a final chance to back out, or allowing them to "cross the line" in the relation knowingly.

In here, the last choice before Lara and MC get lewd lewd is "mention the swimsuit/don't". In this case,  one could imagine it leading to a normal interaction, or maybe an slightly erotic/suggestive one, but one can see the situation playing out in ways that don't necessarily lead to MC doing what he does to Lara latter on. That's why I said that the point - after the breast fondling - when Lara says "we should stop here" would be ideal for a choice, because if you are given a simple "continue/leave" choice, there's no way for the player to understand the "continue" choice, other than "oh, I'm going to be doing lewd stuff with her right and now".

All that comes previously up to that point has built this mood that makes it clear that, if this situation doesn't stop, it'll end with a sexual act. And that's where you introduce the choice, "stop or open the floodgates", and in doing so you make the player the ultimate master of MCs bedroom adventures. At least that's how my ideal for player agency in getting or avoiding sexual acts in choice-based AVNs goes.