A new dating system?

I have always wondered why people engage in sexual activity while dating, if you consider dating an "evaluation" phase of a relationship, it means that you aren't sure if you are ready to love them yet.
I think most people would agree with me that dating is for the purpose of finding a spouse, so it would logically follow for someone to refrain from expressing their love through sex, until they are absolutely sure they want to love that person and be together/get married.
So why is it that people engage in sex and sexual activity (kissing, oral, etc.) before they know how the relationship will end?
It doesn't seem logical to me that sex should be part of the evaluation. If you really love someone, good sex wouldn't be a factor towards finding a spouse, rather a "bonus" that you could have in a relationship. And further more because sex is a function to reproduce, it would make sense to me for people to be careful about who they engage in sex with, I think they should be ready for the consequences of having a child or even a family.
Of course I understand that there are contraception methods, condoms for example are a way to practice safe sex and also prevent child birth, but even then there are risks. And there are many cases where unplanned pregnancies happen, even though they should have access to condoms or birth control pills, these things still happen a lot, and the only real solution to this problem is to be extremely careful who you have sex with.

That being said, I also understand that us humans are subject to our sexual desires, and when we spend a long time bonding with someone of the opposite sex (or same sex if you are homosexual), our sexual desires for the other person become really strong and we feel compelled to engage in sexual activity.
However I don't think this is a good reason to actually start engaging in sexual activity, rather it is just a natural process that happens because we are human, and we should exert self-control over our desires and not let our desires control us.

The philosopher Plato said that the key to a happy life is to let the rational part of our soul (logistikon) rule over the other two parts of our soul, which are our desires (epithymetikon) and our emotions (thymoeides).
So if we follow Plato's advice, we should't let our sexual desires rule over us and dictate what we do, rather we should be very careful about who we engage in sexual activity with.

TL;DR: While dating, why would people engage in sex? Does that help the "evaluation" process?

 

Yeah I think you're mostly right, it just seems to me that it isn't healthy dating if you aren't trying to find a spouse, but that's just my views.
Also having sex can be dangerous emotionally, and it can make relationships uneasy, so it would make sense to me to be more careful about sleeping with someone when you are dating them, but again that's just my views.