Polyamory

I can't see Marty being very comfortable with it

he'd probably knee-jerk assume it meant sanctioned cheating and not like it

maybe if he got talked through it very carefully

but he's very much someone who used to cheat and is very focused on not doing it again, so it'd take a while

Rawne... maybe? if he met the right people

he's not very jealous, but relationships aren't a priority for him

and he's only just, on the Barge, realizing that the people around him are potentially attractive

Thomas would be good at polyamory, as he's fairly casual about his relationships and isn't one for jealousy

I honestly headcanon that he, Crozier, and James Clark Ross had an informal arrangement, at least for a while