Who is involved in the 'fixing the sink sex story'?
Without more details, it's impossible to say who is involved. It could be roommates. For example, if one roommate is trying to fix the sink and there's some sort of sexual tension or situation that arises between them. Or it could be a neighbor who comes over to help fix the sink and then something unexpected happens between them on a sexual level.
I have no idea who might be involved in this 'fixing the sink sex story' as there's no context provided. It could be a couple, like a husband and wife, where the act of fixing the sink creates some sort of situation between them. Or it could be a plumber and a homeowner, and some sort of relationship or encounter develops during the sink - fixing.
Given the lack of information, anyone could be involved in this 'fixing the sink sex story'. It might be a single person living alone who has a fantasy or memory related to fixing the sink and some sexual aspect. Or it could be a group of people in a shared living situation where the sink - fixing event leads to a complex web of relationships and sexual undertones among them.