Antibiotics after sex

So I’m about to start taking antibiotics after sex, the first time being Valentine’s cause we won’t have sex till then.

We typically go multiple rounds and the plan is I’m essentially in bed all day lol

But would I take antibiotics after the very first, or last?

EDIT: the antibiotics are to prevent uti after sex as I recently just had 3 in less than two months. I talked to my doctor and we agreed this is the best course as we aren’t sure why this is happening.