I have trouble cumming without the strain of muscles in my lower body. Wether it be during masterbation or getting oral during sex I rarely can cum without straining my leg muscles, which makes it almost impossible to have sex with my gf with me on top or hitting her from behind(doggystyle)lol. Its rough because I'm an a$$ man favorite position is doggy. All we are able to do is her on top and that can get old for her and me. Its frustrating because the urge is their my penis is fully erect but I cannot cum without straining

Any help would be appretiated am I the only one is this common what could cause this
Its like I can't relax.