Teenage Anakin is set on his first time being with Padme, however he has to deal with hormones and peer pressure and, heck, even his Master is encouraging him to explore this part of life. So his mind adjusts this intention to his proper first time being with Padme, and for now he is willing to try this sex thing (and maybe gain some useful experience!) with one caveat: no penetration for him, that is reserved for Padme. But penetrating him is quite fine!

Bonus points for how it will play out later when Padme comes into picture.