Why not? Though I'm not exactly sure what you have in mind about having a liberal attitude toward sex, I can imagine having permissive views about sex among consenting adults -- views that permit sex outside of wedlock, sex outside of stable monogamous relationships, kinky sex, etc -- and thinking that abortion is generally impermissible except in limited cases involving such things as rape, incest, genetic deformity, or the health of the mother. One might have the liberal view about sex, perhaps because one values individual autonomy. But one might think that responsibility goes with autonomy, making one responsible for one's autonomous choices and their reasonably foreseeable consequences. In this way, one could combine a kind of liberal view about sex with support for some restrictions on the permissibility of abortion. I am not endorsing this combination of views, but I don't see that it need be unprincipled.