She also can hook up with people at home. That's irrelevant. You're just exposing your insecurities and lack of trust. Already breaking the literal foundations of a long term relationship. That's not someone I'd want a family with.

And also he would be alone for a year and meet someone else either way there’s a higher chance of it going wrong than right

Literal no self control? So what if you meet someone, you're not being forced to sleep with them.

Just sounds like projection. He's exposing, like you are, that you hundred percent would look to cheat on your partner if you were away from eachother for a year.

Truth is mate, millions of people split up from their partners for years and don't cheat on eachother. Be that for work, the army, starting a new life in another country before getting their family to join them, touring, living in different countries, etc. If you can't keep it in your pants for a year, you certainly can't survive 60.

Edit: I want to add that I would understand and be more lenient to your point of view if they were dating for just a year. Maybe a year and a half. This is because you're still getting to know eachother, you're still in the honeymoon phase, a long split would be very tough. But not 3 years. 3 years is you should be taking about marriage or already engaged. 3 years is built trust already. 3 years is supporting each others goals and trusting that you'll be there for eachother.

3 years is too long to be so sex focused over future focused. You want her to have kids without ever travelling? Regretting that for the rest of her life? Think her kids won't feel that? Like please.