@Arkandel If your PC is going to cheat IC on another character do you feel obligated to let your partner's player know?
Yes. Any large plot choices that inevitably will affect a character whose roleplay is tied to mine, needs to be discussed with the player of that other character. Otherwise, the risk of drama migrating from the narrative into the OOC far too high. Also, I do not let them know -- I discuss it with them. Letting them know indicates that I have already decided, and am only informing them as a courtesy.
Do you think you are responsible for a character whose roleplay is related to yours if your paths are to separate? In other words do you feel guilty someone else's PC might become quote/unquote unplayable because of your IC choices?
Yes. Again, communication. Before you enter a relationship between characters, discuss what you want from it. Domestic bliss is a nice goal to have IC, but in actually, it's bloody dull roleplay once you get there. I am far more willing to enter an IC relationship where we have decided in advance that this is a slow burn, long term build-up. Happily ever after is not the plot's purpose -- the journey is.
Very closely related to the above, what if the choice that takes a PC mostly off the table is OOC? For instance if you stop being active on my PC's spouse to play an alt with Theno's PC. Do I have the moral high ground to get pissed off?
Depends. You can't ask someone to keep playing a character that no longer interests them, for whatever reason. You can ask them to provide closure, however, so that the story of your character can move on. Their character may file for divorce, die, move out of the country, anything that works for the both of you, and lets your own character get on with their life.
I've been burned on this one a few times so nowadays I create transient characters. They may enter relationships but their basic attitude to life is that people come and go, and while you may be here today, you might as well be gone tomorrow. Enjoy the other person while they're there, and don't plan too long term.
When it comes to TS what's the correct way to suggest it? Do you let the RP become more explicit until the big words come out or you get told no? Do you page the other player first and explicitly ask if they want to do it? Something else?
TS is not my goal when roleplaying but I will act it out if I feel it has narrative qualities; I'm not interested in text porn for its own sake. At the point where a YA novel would do a fade to black I page the other player and ask their preference, possibly a little earlier. Since none of my characters engage in sexual encounters out of the blue, I imagine that I already am quite aware at this time whether the other player is just in it to get off, in which case I am not likely to engage.
Assuming OOC consent between adult players is there anything in an IC relationship, including TS, that you consider unethical?
No. There are things I personally wouldn't want to be part of roleplaying, but what happens between other, consenting players is none of my business as long as it's done in private.