Wrong, firstly women are the protected sex while men are the disposable sex. Men are disposable because sperm, when compared to eggs, is cheap. A man could, in theory, quite easily impregnate a hundred different women in as many days, whereas a woman could only end up being impregnated the once in that same time period. As such our species has ever taken the death of large numbers of women more seriously than the death of large numbers of men - a shortage of men merely requires men to take on more sexual partners in order to re-populate the species whereas a shortage of women is where we start looking at the decline of our civilisation, or to put it another way, men are easily replaceable, women are not. That's why whenever a women is in pain everyone rushes about to do something about it, whereas when a man is in pain he's told to Man Up and stop whining.The reason for this lets take a step back in the past in human past time where males are dominate and women was not no this is not a sexist attack this is just how history Is and was. There are other reason a female was not allowed to go to war due to distraction of seeing the opposite sex being killed outright could be more horrifying then someone of the same sex and the horrors of a woman being captured that. Or most men back then justified there manliness by saying woman are weak and such this could be the very reason women are not allowed to be space marines hence look at IG no real speaking of female IG as they are all male even in the fluff there is very little on female IG command and such.
So it makes sense that if becoming a Space Marine sterilizes the recruit, that you limit it to men.
Secondly women ARE physically weak in comparison to men (in general biological terms, I imagine Jodie Marsh could quite easily kick a stick-thin emo boy's arse even if he fought back with all his strength rather than holding back and taking it because it 'isn't right to hit a girl') - have you not noticed that even the Olympics to this day still maintain separate men's and women's events? Do you think women athletes would appreciate you campaigning to eliminate this 'sexism' and thus making their chances of picking up some Golds (or even being picked for the national teams) a remote possibility? Did you know that the required fitness levels for women in their early 20's joining the US army is the same for male Army pen-pushers in their 50's? They had to be reduced in order to let more women in, which is, as far as I'm concerned, an insult to those women that could match their male peer's physical requirements* and a deliberate policy of putting weak links in soldiers lines of support.
So again, if part of the Space Marine process involves massively bulking up on muscle, it makes sense to restrict it to the gender that is more prone to putting muscle on in the first place.
*And 40K's version of these sort of women is the Adepta Sororitas, who are usually described as being hard as nails even when out of their power armour.