The issue of consent becomes as fuzzy as it occasionally does in rape cases. It's easy to be clear at the extremes, but it can get very fuzzy in the middle, especially if either or both people change their minds about what happened after the event.
That's not going to be a popular idea either, I suppose.
Maybe condom packets could come with pre-printed indemnity contracts which could be signed and exchanged in the presence of a lawyer before the condoms are used?