As others have said so well, solve this BEFORE getting married, not after. Getting married won't make this any easier, it will just increase the consequences, and being polyamorous won't necessarily help your sex life. Polyamory will bring its own problems, so you'll need to be good at resolving things together.
Look at opening your relationship as something akin to having a child together: it needs to be something you both want, it will be a challenge even if/when you've done what you can to be prepared, and you can never fully take it back.
If you're wanting to get better at satisfying each other sexually, take a class together that is focused on that. Communication is key (and something everyone needs to continually work on, myself included). If you're able to satisfy yourself sexually, have you tried having your boyfriend hold you while you do so?