A penis plug would probably cause injuries during normal sex, unless you had sex slowly and carefully, no hard of fast thrusting. It would depend on the size of the external part of the plug.
My guy modified a cum-thru penis plug by cutting off the external part, inserting a string through the hole and gluing it in place which also plugs the hole. The plug stays entirely inside his penis and also prevents cum from shooting out. When done, he pulls on string to take the plug out. The plug also helps keep his penis semi erect after he has an orgasm which prevents slipping out before I am done.