Currently I am undecided if I still be-lie-ve in monogamy, and really I think the fact that this thought comes after a long life of having this strong moral high ground regarding monogamy gives me a bit of paws pause...

I have written about marriage and relationship right above this blog post ;) You'll see my stance on it there.

A life with surprise sexual encounters is kind of fun and enjoyable, and why should a man or woman feel ashamed of enjoying the pleasure that comes from sex, or a diversity of sex?

Indeed, a good question. Why? I don't see any reason why I shall feel ashamed to combine the most optimal "the man I decided to be with and trust (and vice versa) and I give each other pleasure". You seem to talk about Puritans (and similar streams), where sex and enjoying it is being frowned upon. That is not overall the case with Christians on the whole.

I have read comments on youtube in large numbers where happy couples who say of themselves that they are Christians report from their fulfilling intimate relationships, lasting decades. I think you could read happy examples in order to make your balance sheet.

For myself I can only say that the best and greatest intimacy I had and have was with my husband and that we did not lack any joy and pleasure with each other in that regard. Actually, our intimacy is one reason for lasting long as a couple. Yet it becomes less of an important thing the more you started out in the first place in having explored and enjoyed the sexual part.

... Let me formulate a question: Are you willing to consider that the probability of having a superb sexual relationship is potentially there because of monogamy? What if the monogamous relationship holds up the even greater possibility of superb sex? And also: Would you think that having a long term relationship to just one man and one woman excludes automatically "surprising sexual encounters"?

Let's turn around the rule of monogamy and formulate the opposite, using the same style:
"You shall have changing sexual partners" - how does it sound?

edit:

I found this quote in the Old Testament:

Bless your fountain. Rejoice in the wife of your youth, the lovely gazelle, the graceful chamois! Her caress makes you perpetually drunk, intoxicate yourself with her love again and again! Why should you intoxicate yourself with a stranger, embrace the bosom of another?