I can actually see Anders being apprehensive about getting involved, not the spirit. Yes, he's a womanizer, but he's also always had a lot of concern for other people. And according to what we know, he's literally fighting to keep the spirit/demon in check. He may be worried the demon could take advantage of the, um,
moment of distraction and take control. And perhaps hurt Hawke in the process, or be unable to take back control from the spirit.
Also, the full achievement descriptions are
here. It looks like there's two generic romance achievements, to start and complete a romance with a companion. Not specific achievements for each potential romance.