"Once any man can be a woman all legal sex based protections of women dissolve, how can you sue your employer for unfair dismissal because you got pregnant if men get pregnant too?
How does men getting pregnant affect that, exactly?? Pregnancy legislation will continue to cover, er, pregnancy."
It's actually really simple. Say you have a man and a woman for a job interview, the woman is more qualified/experienced. The woman is of child bearing age and after a carefully constructed bit of small talk by the interviewer reveals she is recently married and has just got the keys to her new home. The interviewer decides to go with the male.
If men can get pregnant too, the women loses the argument that she wasn't selected because it was assumed she'd want to bear children quite soon. She is won't be able to challenge the recruitment process because "men can get pregnant too" therefore the protection that sex discrimination should provide is no longer required.
This happened to a friend btw and she won at a ET and the company admitted they didn't recruit her for the above reasons.
(But we all know men can't get pregnant, hell my 3 year old nephew knows that)