I think that for me, having a wonderful new person highlights how wonderful my established relationships are. I know that if my relationships were not solid, loving, supportive, and trusting, I wouldn't have the freedom to make new connections with others. The outpouring of love and desire for the "old" love/s isn't misplaced sexual energy, it's a gush of appreciation for the rocks in my life.
Plus, I get weirded out at the idea of what other people see as sexual spillover, or sexual energy that "belongs" to someone else. If Fly is all frisky because he got a promotion, is that sexual energy directed at his boss? I think I would be offended if someone I love doubted the sincerity of my desire, or took it upon themselves to divvy up *my* sexual energy and assume what I'm sharing with them was meant for someone else. I have sex with people I love because I love them, not because I have a horny itch to scratch.
When Fly is having a great time having good sex with someone else, and he comes home and is affectionate and physically interested in me, I enjoy it. I know he's happy, feels good, appreciates that he and I have built a foundation that gives him the freedom he didn't have in other relationships. It's less that we're bringing other people into our bedroom, and more like we're reconnecting the energy between the two of us.
With Moonlight, it's a little different because she's pretty mono, so it's more of a one-way street. But she recognizes that when I'm happy in general with my life and partners, that I'm more likely to want to be sexually intimate than if there's conflict. She doesn't have any issues with spillover - she knows that I love her and that when we have sex it's because I want to be with her.
Sorry, this all got more rambly and stream-of-consciousness than I intended.