As someone polyamorous, there's definitely something to this. If I already have one partner, I feel more confident in approaching new potential partners. Knowing somebody already cares about you and supports you can go far.

What I don't understand is the cheater mindset - the idea of going after someone behind your partner's back. Especially these days, where people know about polyamory. If your partner's not into it, then you have to make a choice - do you want to be their one and only? Then stick to that promise. If not, then the honorable thing would be to be honest and accept the break up.