Does it help at all to know that sex is completely and utterly different with each partner? Yeah, it's made up of the same components, but it's still unique.
To compare to two other activities I enjoy:-
Playing squash - I've played squash with a lot of different people in my life, and most of the time one game of squash is the same as another. Yeah, different opponents present different challenges, and require different tactics, but mostly it's just about trying to play the best game I can, and that mostly goes the same no matter the partner.
Singing with someone else - This is much more about the person you're singing with. Every new person I've sung with is a new experience. You find your own tempo, rhythms, how your voices work together. It's about creating something shared together. Even singing the same song. Yeah, the notes are the same, the melody is the same, but the combination of our voices sounds different.
Sex for me is a lot more like the singing than the squash. It's something you learn to do together. You find out each others tempo, rhythms, what works with each others bodies. Like the singing, the individual actions, the "notes" might be the same, but the experience as a whole is different.
I've had sex with a number of people before meeting DP, but I've never had "sex with DP" with anyone else. And the same is true for DP, she's had sex with other people before me, but I'm not jealous of that because it's a different experience to what she's had with me.
Sex isn't sacred. Dogs do it, rats do it, slugs do it! The sacred part is the connection between the two of you while you have sex, the unique experience you create between you, that isn't possible with anyone else, because it's unique to the combination of two unique individuals.