Ascended characters are definately on a par with Deathwatch marines in combat and will outclass them in almost every other sphere of activity which gives you the option of including female PCs. After all, remember that the Deathwatch is under the auspice of the Inquisition. It is therefore entirely appropriate to mix it up with other types of character.
Deathwatch is SO MUCH MORE than a monochrome combat kill-team. their primary task is investigating xenos breeds for exploitation or extermination.
The simple fact is that space marines are ALL MALE. Its pretty much the only solid canon that GW has ever issued (not mentioning the female space marine miniature released in the early days http://www.coolminiornot.com/169720 ...that was just 'a female in power armour' apparently).
And as mousemuffin says, why should we let that spoil our fun? http://www.coolminiornot.com/159605
Seriously, all marines are male.
But if you want to change your 40k to have female marines, go for it!!
Still, the 'female space marine' topic crops up every now and then and typically guarantees over 100 replies and a flame war! Hehe...