You can have sex with anyone (if they are an adult) applies to all humans... If you have sex with someone else that woud be cheating... LBGT sex is common as there are LBGT in western countries...
By western countries I'm assuming you mean North & South America. I'm from the USA. Here gays and lesbian aren't exactly considered normal but it seems to be publicly seen more and accepted easier than it used to be. Media and such are just (within the last couple decades) getting into displaying happy perfectly fine gay people, but their are still a lot of people who just can't accept it as a way of life.
Yes here a person can have sex with whomever they wish as long as that person is at the age of consent or if your 18 they're not younger than you. (although I'm sure it still happens)
If you're married it would probably affect your life unless you and your partner are both in agreement with this, but in most cases being married means that your partner is the one and only person you want to be with.