Husband says I tighten my legs and push while he is on top.

So, I know this is probably an odd question. I like to be on bottom. However, hubby has a hard time raising because my legs "tighten" and he has a hard time being able to get the thrusts he needs. I am not sure if it is a natural reaction and when I get closer to Orgasm tighten, but I was wondering if anyone else has had this experience. What can I do to keep those muscles relaxed? I do not feel when my legs tense up. I love being on top don't get me wrong, but bottom is and always will be my fave position. Also, I have heard bottom has more of a chance for conception and whether it is an old wives tale or truth, as long as it is not hurting me or the child, what could it hurt lol. Also, would spooning from the side be close to being on the bottom?