Honestly not everyone can offer everything sexually to a partner. This is reading more like an open relationship than poly. You are her lover, he is a dominant fuck friend. You can each give her what she wants and still be strong individually.
And boy oh boy. If you feel your masculinity fading at 30, you are in for a lot of trouble. Age has nothing to do with being masculine, its in your head. Once you learn that, the idea of being "old" kind of goes the way of the do do.