The problem I had when I was in that situation was ...a) having to mention it... b) the forced dates....or obligatory dates....ok I'm here what do you want to do ... Wait hold that thought I just got a text....reading reading ..giggle giggle...ok what were you saying.
So my point is a schedule could help carve out time but that doesn't drive desire, excitement.....and its exsitents is to simulate equality.
Why not find a gf of your own .....the thought that by her adding partners will improve the frequency of you getting laid is crazy. In your opinion you're are already dissatisfied with that was that the case before opening up the marriage or after the gf? Either way it's not going to get better ...time is finite. 15 yrs of that.
Have you considered this option?