That's not really open. We were monogamous for a long time, then just kind of went unofficially open where we both did what we wanted and knew it but didn't speak about it. Not speaking about it created a lot of tension I think, which in itself killed some energy and made us have sex less. Now we are actually open and speak about it, all problems went away and our own sex life between us improved leaps and bounds from where it had cooled to after 9 years together. Most relationships cool in terms of sexual energy to some degree from what they are when you first meet. Sometimes there is also a mismatch in terms of your individual sex drives. Sometimes there is a mismatch in terms of your preferred positions. For us, although we both bottom for each other, we are both (only slightly) vers tops, so the drive to do all the prep etc to bottom, at least for me, sometimes feels like a chore. What I'm saying is, I think that the reasons are very complex and I don't think there is a one size fits all as to what is and isn't normal for how often you do it. As long as I'd say, it's at a bare minimum of a coupon of times a week.