Well, have you tried - for lack of a better term - training traditional guy to please you sexually? He may not have the experience to know how to last longer and satisfy you.
Also you are responsible for your own sexual satisfaction. You need to tell him - gently and with tact - that you are not satisfied and here are some ideas about making changes. And make sure they he's a full partner in discovering what works for both of you. It should be a joyous, fun, silly, intense dance of discovery.
Poly acknowledges that one can't be everything to another. (Mono relationships can also acknowledge this.) But starting a relationship with bad sex and expecting the sex will never get better Iis a recipe for disaster for that relationship. You deserve good sex in your relationship(s) and that is very achievable.
Hopefully traditional guy is open to learning and experimenting. If he is not, if missionary PIV is the only 'real' sex, if he is too insecure to listen, learn and experiment with you, dump him immediately. A partner who asks for what he/she wants, experiments, and is talks openly about sex is a great gift. I've been blessed with partners like this. You can be that partner for traditional guy.