Because we selflessly want to introduce women to a new sensation that might be something they love.
Ok, so that's maybe .5 percent of us.
How to get your girl into Anal sex is easy.
1. When you are eating her out, use a finger to tease around her asshole. No insertion.
If she stops you or doesn't seem to enjoy it, skip to step 8.
If she seems to like it then move on to the next step.
2. Next time, when eating her out, start to finger her pussy and work your way around to, gently, toss her salad (eat her ass).
If she stops you or doesn't seem to enjoy it, skip to step 8.
If she seems to like it then move on to the next step.
3. Same as 2, but remove the word gently. Really work your tongue in there.
If she stops you or doesn't seem to enjoy it, skip to step 8.
If she seems to like it then move on to the next step.
4. When you are eating her out, use a finger to tease around her asshole. Make sure finger is nice and wet, either from her pussy or from your saliva and slowly work it in, just a little.
If she stops you or doesn't seem to enjoy it, skip to step 8.
If she seems to like it then move on to the next step.
5. When fucking her, doggy style, tease her ass with a wet finger and gently insert it, just a little. Don't do more than that. Just keep it in a little.
If she stops you or doesn't seem to enjoy it, skip to step 8.
If she seems to like it then move on to the next step.
6. When fucking her, doggy style, finger fuck her ass.
If she stops you or doesn't seem to enjoy it, skip to step 8.
If she seems to like it then move on to the next step.
7. Ask if you can fuck her ass.
If she says yes then you are there. If she doesn't, then move on to the next step.
8. Give up. It's not her thing. Find someone else, or be satisfied with what you have.