Forget sexual compatibility and "trying out" their partner before marriage. What they need is to get it out of their system and treat it more casually, with no thought to marriage. And marry later in life.
I'm only speaking from my own experience, but I would think that most people who try it for the first time would think it's great and they're compatible with their partner. Your first time is literally the best (and worst) sex you've ever had - at least with other people. You usually need a lot more and multiple partners to get perspective if you want to compare good and bad sex.
Marriage should only happen when both partners are more mature. Right out of mommy and daddy's home and into a marriage isn't mature. They should live on their own and find out who they are before thinking about sharing their life. When you're ready and find somebody you want to share a life with, not just to get laid.
Same as sex goes for dating. Whether or not sex is involved, dating needs to be more casual. When I was a JW (and maybe this was local), asking somebody for a date was practically like proposing. Maybe that's why I'm still shy at asking girls out to this day. Before long, couples are questioned about intentions and pressured to marry. And quite frankly, under the watchful eye of chaperones, I really don't think you ever see the true person you're dating. So, generally, boyfriends/girlfriends ended up marrying. If they broke up while dating, it was scandalous with what seemed like the whole congregation taking sides.
Sex should be more casual. However, there is something to be said for marrying your first. It's a bond you share. Something special that you've shared with no one else and a major experience you have in common. Although that is hard to accomplish. Because finding the right one is so hard and so is waiting. IF you wait and find a person you're compatible with, it's probably an awesome bonus. I remember my first like it was yesterday. It was yesterday in fact - just kidding... The back seat of that tiny little car at the drive in with a worldly girl... Both virgins. Ah, memories...
As for "trying it out" before marrying somebody so you aren't stuck with a bad sexual partner, well, I don't think I'm the only one to say things can change after "I do". And that great, frequent sex in the dating phase isn't so great and frequent later on. When I was a JW teen, I imagined sex all the time once I was married.
I married after leaving the JWs and had no guilt about having sex before marriage. I even lived with my ex for a couple of years before getting married. Soon after the honeymoon, the frequency declined. The quality declined. And there were just some things she no longer did as willingly as before. They became Christmas and birthday or "I'm really drunk" things.
