It was poorly written, to the extent of obscuring its message. Imperial have acknowledged this.
Well not really, because the only people questioning this on Twitter are TERFs. No normal person is going to see that advert in the Metro and think they're looking for men who have periods. There's nothing wrong with the wording and nothing for them to hold their hands up to. There's a distinct difference between causing confusion (to TERFs) and being confusing. And this is an example of the former.
In a trans-inclusive society, it's not at all unreasonable to assume that menstruating transmen were being sought. (I'll just put this here again. www.dailymail.co.uk/femail/article-5500565/Transgender-man-campaign-PERIODS.html) I'm not seeing why it's transphobic to think that??
(And just to politely point out, TERF is considered a slur, and not really conducive to respectful communication.)