Why do many "pro-lifers" make exceptions in the case of rape or incest?

Often the idea is that when people get pregnant they originally had a choice to risk doing so by having sex. If a woman is raped she did not consent to taking the risk and so in that instance pregnancy can be seen as being forced upon the woman. Therefore, some see it as a valid reason to allow the woman's right to life without pregnancy (since pregnancy means not working for a period of time even if the woman gives the child up for adoption plus physical distress) to outweigh the unborn child's right to life. I personally think a life is a life is a life and instead of government's providing to cover abortions (like in Canada) they should provide money so women can afford being pregnant and then give up the child for adoption.