He might support her right to choice because he recognizes that her needs and life goals are as important as his, and that in a case where the pregnant person wants to terminate and the partner doesn't, only one person will be fully satisfied - and in a case like this, it's the person who will be pregnant and give birth (or not). You're not being forced to believe in it or like it, but to recognize that supporting, broadly, someone's legal ability to terminate a pregnancy is simply the right thing to do whether you like it or not. For another example, I don't believe in organized religions that demand women be submissive to men or be fruitful and multiply, but I support other people's right to be submissive and have children.