Pussy getting tighter on orgasm

I have been trying to Persuade my wife Carol (Late thirties) to have sex with a friend of ours Martin (23) She almost did in our car but they were disturbed. Then she said she was not sure if she wanted to or not. It would be to risky for her as she knew I wanted him to have her bareback.
Last week we were out for a take and we met him. When we got home I gave Carol a blow job and between licking and sucking I talked to her about Martin. How smart he was, How I knew he fancied Carol. That I thought he wanted to fuck her. I told her I had seen his penis and It was bigger than mine.

When she was very excited I ask her, would she let him put it inside her, just so that she could feel what it was like to have a bigger one inside her. She was well worked up and said she might if he did not move when inside her and pulled out when ask. I had two fingers inside her and was sucking her at the same time and she started to cum. I have never felt it before but her pussy tightened up a lot and squeezed my fingers as she had a very big orgasm. The next night I put on a rubber and we are fucking. I talked about Martin again, and said I wanted to see him put his penis inside her, just for a minute. This time her Pussy Squeezed my penis so hard it made me cum and nearly pulled the rubber off. I cant wait to get Carol to (let him just put it in her for a minute, just to see what it feels like). but I am not sure if she will let him or not. Is this normal when women think of big cocks to have their pussy get tighter?. Have other women had this happen. I would love to know.