Hello there! And welcome to the site. I'm pretty new here too, and have found it very useful, and I hope you do too!
I don't know much about weight loss surgery, but I had an idea on what could be causing your sex drive to diminish.
Are you taking medication for pain? A lot pain medications when taken for extended periods of time can cause depression. And sometimes (a lot of times) with depression comes the loss of sexual appetite.
Sometimes it's hard to know when you are depressed. And after a while, it can start to feel normal to you. So you really have to take a close, deep look at yourself, and your life, and maybe see a doctor about it.
If you find that maybe you are depressed, there are depression medications out there that can help you... But... A well known side effect of depression medications is the loss of sex drive.
So it kinda seems that you lose either way, right? Maybe not. Talk to your doctor about it.
Good luck with your surgery, and everything else!
Hang in there!
~qriosity~