So just go ahead and do it. Female SMs don't break anything mechanically.

Why on Earth would you put you and your group through that?

*dons flameproof suit* What's wrong with feminists? Supporting the rights and perception of women doesn't mean they aim to worsen the situation of men. Aside from removing the advantages, obviously, but that's a necessity of creating equality.

I think the term has been unfairly blurred somewhat by (1) also being applied to some women who are sexist against men and (2) from certain people campaigning against this sort of change.

If a feminist has trouble playing a Space Marine due to their gender, this is probably because the design of the Astartes comes across as unintentionally sexist, as the setting did not really gain anything out of this arbitrary limitation (especially as it makes zero biological sense @ zygotes). So, depending on the player's own feelings on the subject, it could feel as if they were "supporting" this situation, and so betraying their own cause - especially as the rarity and depiction of female heroes continues to be an issue in games and movies these days (or rather, has just recently started to kick up an overdue debate).

However, just like with skin colour, it's safe to say that this issue was simply not on the writers' radar - you can see the same problem in 40k artwork that is almost universally white males, simply because the artist was a white male. And it's not as if GW did not attemt to compensate for this by balancing the all-male Space Marines with the all-female Battle Sisters, actually presenting them as their female counterpart. So what I'd do as a GM would be to tell said feminist of this "mirror organisation" and making sure to present them in a suitably badass way as well.

Also, I never understood why people "want to play DW" but "Dun' wan be Space Marine". It's cropped up a few times, the issue being that Deathwatch was always intended to be Space Marines only, and it plays as a game with only Space Marines, balanced for all the players to be Space Marines.

I'm guessing it's either peer pressure, in that the group as a whole wants to play Deathwatch, or simply the game's presentation of playing legendary heroes in epic OTT battles. You don't really get this in any of the other games - or, in other words, Deathwatch is simply "the next best thing". Just like people modded Space Marines into Dark Heresy before Deathwatch came out.

This is the disadvantage of not building the RPG as a universal ruleset, but rather segregating it into different rulesets of different power levels and gameplay styles.