Sorry i missed your questions and they are both really good ones, and I'm glad you like the direction we are going, this game is much more about romance than sex and in order for romance to build, and this might seems counter intuitive, but it's important to keep the lovers apart, at least at first and to add lots of challenges and problems to make the payoff 'real'. Red has done such a great job layering the personalities of these characters, making them real nuanced people and not just there as a toy for the player. In fact most of the time the NPCs are pursuing their own goals independent of the player, and an NPC might pursue you or try to date you for their own reasons, though this usually happens if you flirt with several of them and lead a few on, then ignore them.
Anyways as to the two questions:
1.) There is an 'end game' section for every relationship that ties into the reasons you were brought to the island and why your character is starting their life fresh. I really don't want to give this surprise away because it should be a great story moment. And yes there is a Relationship Point threshold for every character where they will consider you a partner/lover. This level is different for ever character and some will either ask you out, or you will get dialog to ask them out, exchange 'i love you's and things though some shy characters have very low thresholds and might not say much and assume you are an item and if you contradict this they will get upset. There's already at least one "Are you cheating on me?!" "No...I didn't think we were a couple!" scene lol.
2.) Every character is technically bi-sexaual, but some gay characters like Michelle and Lisa are extremely hard for opposite sex people to date. Same for some of the strait characters, but you can date them but you have to make them really fall in love with you so that they over look you gender which is easier for, say Michelle who is very emotional and cognitive and also rather submissive, vs. Bruce who is much more body focused and primal/carnal. It's not impossible, but you would need to play their quests almost flawlessly.
wow that was a long reply, hope that answered everything