Because the burden of responsibility is simply not the same in the choice to have sex, the choice to have unprotected sex and the choice to bring the pregnancy to term. The large portion of the choice AND responsibility lies with the woman who decides who she will allow to enter her body(unless we're talking rape), which form of birth control to take(which there are many forms) and if she will abort, give up for adoption or keep the child.
To me, this is just as simple as the "submission" issue. Be wise in your choice of who you allow to enter your body, and then act accordingly. First, if she's even spending her time with a shiftless, no good, unfit type of guy in the first place, then that shows where her mind is at. Because a quality woman should only spend time with quality dudes. Quality being someone who has a stable source of income, is financially autonomous and is of good character. That's a first strike.
Also, a woman who allows a man she is not in a committed relationship to enter her body without a condom and/or without being on birth control is either either being reckless or extremely negligent. Or even if she is in a committed relationship, it goes back to the first one-why be with a bum in the first place? That's a second strike.
On top of that, if she has unprotected sex with this deadbeat, then she has decided to make a reckless decision that she knows, in her hearts of hearts, is not the best decision for herself or the possibly baby she may be impregnated with. And then, if she decides to take that baby to term, she is deciding to bring a life into an already lackluster scenario with two less-than stellar people as parents. Third strike.
I am not downplaying the dude's responsibility or even letting him slide for him being less than a stand up guy. But in this scenario which mirrors the basics of any romantic interaction, the woman holds more responsibility and choice for the pregnancy.
Peace