Why is this? Though I'm not certain if certain pro-choicers believe these are the real motives of prolifers, it's much easier to insinuate evil motives and then write off individuals with these evil motives than it is to take the argument of someone you disagree with seriously and then show why that argument is wrong and why your position is right. This is especially true for pro-choice movement which is struggling to find a foothold in a storm as they are currently stuck between a rock (the scientific evidence of the unborn's humanity, the feelings of post-abortive women, and society's belief that abortion isn't morally right) and a hard place (admitting abortion is "bad" inevitably leads to the question of why is abortion "bad").
Prescribing evil motives to prolifers seems like a much easier task (or at least a good short term strategy) than dealing with the problems the pro-choice movement is currently facing.
The two motives (that I've seen at least) prescribed to prolifers are either that 1.) prolifers want to punish women for having sex and/or that 2.) the real goal isn't saving unborn children but making birth control illegal and preventing procreative sex.
At Alas, a Blog, Ampersand provides a chart which attempts to show that the policies which prolifers advocate are more consistent with the belief that women should suffer consequences for having sex than the belief that abortion is "exactly the same as child murder."
Amp cherry picks policies (why no mention of opposing the killing of human embryos for their stem cells, opposition to assisted suicide or bans on tax-funded abortions?), creates some major strawmen (which prolife organization is opposed to an HPV vaccine and which prolife organization wants to keep abortions for rape and incest legal?) and incorrectly insinuates that partial-birth abortion has a lower risk of injuring the mother even though there is absolutely no evidence for this. However, what's most tragic is his complete inability to take the possible varied motives behind a variety of policies seriously even though he says, "I really like to assume the best of everyone, even people I disagree with. And I try hard to take what opponents say, at their word." The chart that follows his statement is strong evidence that he really hasn't tried that hard to take the varied positions of prolifers on a variety of policies seriously.
Molly at Molly Saves the Day makes a similar argument when she states,
"Guess what that means? It means it's NOT ABOUT LIFE. If abortions should be illegal but there should not be forced marrow donations from living donors, the distinction isn't one of life, it's one of responsibility."
This same kind of obvious ad hominem attack is also clearly seen in a Salon article featuring statements from NARAL New York's Cristina Page who says, "The anti-choice movement has become a religious movement, and because of that, their interest isn't in reducing abortion. In fact, reducing abortion has become problematic for them, because they want to strip Americans of using birth control, in effect to change the entire family structure."
Cristina's main schtick isn't trying to prove that the unborn aren't human beings or that unborn human beings don't deserve to be protected by law but to attack the motives of prolifers by claiming that the real goal of the prolife movement is "the creation of a society in which the only acceptable reason for sex is procreation". In the Salon article, she also asserts that the prolife movement, besides wanting to ban abortion and contraception, wants to "make child care impossible" based on nothing but the web sites of a few organizations that are against abortion mentioning a study from the National Institute of Child Health and Human Development which seems to have found some correlation between child care and aggressiveness in children.
I think these attempts to unmask the supposedly evil motives of prolifers will ultimately fail to persuade the large majority of the American public. The hardcore pro-choicers will eat this up for breakfast, lunch and dinner but most people who know and interact with regular-average-joe-contraceptive-using-kids-in-daycare prolifers will have a hard time believing that the main motive of behind their friend or co-workers opposition to abortion (especially women who are prolife) is to punish women for having sex.
Do some pro-choice people really think the prolife position of wanting to save unborn children is so nonsensical that they have to come up with other motives for us? Or do they really see us as vile sexist minions trying to control women? I don't know. But it's hard for me to understand why anyone would think this kind of strategy would work.