I can believe such a ban would be ineffective if it was trying to keep people from something they were trying to get.

However, this is about banning people who are trying to give something, and who are presumably concerned with actually helping.

It wouldn't surprise me if some men who've done MSM give blood anyway if they have good reason to think they aren't infected and especially if they have rare blood types.

However, at least some people are habitually rule-abiding.

This is all hypothetical, though. I don't know how reliably people obey the rules for blood donation, or how important all the rules are. The whole thing is on the honor system, and there are a lot of restrictions.