Cock in Cunt
You have one of your most sensitive parts inside one of her most sensitive parts. With each movement inside of her you can feel the walls of her vagina, the muscles and flesh.
The vaginal wall is an incredibly strong muscle and she will probably use this to some extent for her own pleasure during sex. You may also feel the contracting and relaxing during orgasm.
Her vagina spasms and this will clench you to some degree but not always, so don’t use that as a benchmark. Some women do have great control, and those who do are likely to have better sex and orgasms in general.
If you don’t feel any clenching, don’t panic. You may know already by the feel of her vagina when she’s not far away from orgasm.
She may let you enter further, or the overall sensation may be “just right” for you both. Only you can tell, so pay careful attention to her sexual body language and what’s going on inside her.