Too fucking stupid:
No, that's just a lazy teleological argument dressed up as wisdom.
The purpose of a thing depends on whose purpose you're talking about.
The biological function of eating is nutrition. That doesn't mean every meal is consumed for nutrition. People eat for pleasure, culture, social bonding, stress relief, bodybuilding, religious observance, and a hundred other reasons.
The biological function of legs is locomotion. Yet people use them to dance, kick soccer balls, climb mountains, and sit with them propped on a coffee table while arguing on the internet.
Likewise, sex has several evolved functions and effects. Human beings engage in it for intimacy, bonding, pleasure, emotional connection, stress reduction, recreation, and, sometimes, reproduction. The existence of contraception alone should have tipped you off to the fact that most people understand this perfectly well.
The funniest part is that your own examples undermine your argument.
The purpose of driving isn't to crash, but crashing is a foreseeable risk of driving.
The purpose of eating isn't to be poisoned, but food poisoning is a foreseeable risk of eating.
The purpose of leaving your house isn't to be assaulted, but assault is a foreseeable risk of interacting with other people.
Exactly. A risk is not the same thing as a purpose.
Pregnancy is a possible consequence of sex. That does not magically transform the purpose of every act of sex into reproduction any more than a car accident becomes the purpose of driving because it can happen.
You're trying to smuggle a moral conclusion into a biological observation:
"Sex can cause pregnancy."
True.
Therefore:
"Anyone who has sex is consenting to pregnancy and forfeits the right to make decisions about their own body."
Not true. That's the part you will never actually prove. You'll just hope nobody notices the leap.
Human beings invented contraception, abortion, fertility treatments, vasectomies, condoms, birth control pills, and IVF precisely because we don't treat reproduction as the sole purpose of sex. We've spent centuries developing technology to separate sex from reproduction whenever people want to.
It's remarkable how often these arguments collapse the moment you ask a simple question:
If the purpose of sex is creating children, why do millions of infertile couples, post-menopausal women, sterilized people, and same-sex couples continue having sex?
The answer is obvious to everyone except people desperately trying to turn a biological capability into a moral obligation. Human beings are complicated creatures. They built particle accelerators, wrote symphonies, split the atom, and somehow still produce tweets that wouldn't survive a five-minute conversation.
The purpose of cars isn't to crash.
The purpose of food isn't to poison.
The purpose of leaving your house isn't to get assaulted.
The purpose of sex IS to create children.
Hope that helps.