I agree with you that
Larkin sounds extremely feminine, and I ordinarily like unisex names much better on boys! I'm reminded of a lark, which is a songbird - traditionally associated with girls. And the -in suffix is sometimes used as a diminutive nickname, intensifying the femininity. To be honest, I don't like the name very much, but I can't even imagine this name on a guy. My sister had a (female) friend named
Larkin in high school.