bfx, the important thing is to have fun now, to deepen your relationship now, and continue being honest and open about fears and feelings. “Now” is all that’s real.
I went through three long relationships and two (too) long marriages before I stumbled across my current partner and and for the first time didn’t want to be with anyone else.
I was sixty-fucking-three when I got it, that monogamy is just wanting to have the best time with someone you like — not externally imposed.
Prior to now in my life, monogamy was just another word for controlling. If monogamy is not based on the desire and joy in being together, then it’s control. Good for her that she’s exploring her feelings about sex and sexuality. She may want to do more exploring than you feel comfortable with - if that’s the case, figure out the roots of your discomfort- which is probably insecurity, which leads to control.
Still, time with others is time not with you.
Having a “relationship” or an imagined “future” with someone does not replace the necessity of also having to have a life.