My two best friends are dating each other
This is one explanation that has literally nothing to do with them, and if you decide this is the problem, you might want to sugarcoat your answer a little when you tell them.
This is one situation where I would probably say that your friends are jerks.
If you feel that this is the reason you don’t want to be around them, it is OK to let them know – although you shouldn’t expect that they’re going to change just because it makes you uncomfortable.
Most people, when entering a relationship, will want to be affectionate with their partners.
Either things will get and they’ll stop asking you to hang out, or they’ll confess they’re into you, too.
If one of them likes you but the other just sees you as a friend, and they’re not into the idea of an open relationship, basically, things are going to get awkward. But if they bring up the idea of an open relationship, this could be the solution to all your problems!