Originally Posted by BlackUnicorn
Welcome! You are definitely not alone. Do a search on triads and threesomes and you'll see.
First order of business - you need to permanently disengage from all sexual behaviours you are not comfortable with. Trust me, it will save you years of therapy.
The primary lady is having way too much control over you. Disengage!
Second order of business - Talk to her. Find out what she's uncomfortable with. Talk to them both. Work through it. It sounds like she's being jealous or having issues sharing, which is normal, but not healthy. If he's oblivious, you need to make him aware so he can address it.
I couldn't imagine dating someone for a year and a half and never sleeping in the same bed as them. Obviously it is causing you a great deal of turmoil and you need to address it one way or another.
Find out if this is a hard limit for her. Is she going to leave him if he does this? Why? What risks do sleeping in the same bed pose that having sex (or threesomes!) don't? Does she think he'll love you more than he has already in the past 1.5 years?