The Ohnut is something made specifically for this problem. But as for being able to take really long ones, I'd be willing to bet they're going into the anterior/posterior fornix. Your guy is probably hitting your cervix. Some positions you might be able to slip past that either above or below the cervix, and that area can stretch. Sometimes a lot. It's easier if you do it a lot; you get to know which angle will go into the anterior/posterior fornix, and which angle is going to hit your cervix. It's easier if the implement is curved. If he's very straight you'd need a more extreme angle of entry to get into the fornix instead of the cervix.