I have the vanilla code slightly modified but the normal behavior is always to run a check at the end of the day for anyone working the same position and sleeping in the same location. It may be a combination of jealousy or sexual preference.
Constants might theoretically be messing with it but I haven’t looked at it in a long time to recall if that changes any part of that code.
Regarding changing relations, I would recommend bringing them into your party to have them fight together or having a 3-4some (that should be the biggest relations increase). A few orgasm together should get them to a positive relation. After they have a positive relation towards each other (and/or their sexuality has shifted towards bi or even gay) they shouldn’t have negative relations afterwards.