A man's anus is shaped differently then a woman's which is why many men... both gay and straight find pleasure in this, also they have a prostate gland which allows for an orgasm without penile stimulation. Because a female has lady parts our anus/colon is shorter and curves more, around the vagina. To start, if you're curious... you'd want to use positions that align your body straight.... doggie is a bad position IMO because you're tense and you have no control over your partner's thrusting... and you really need to be able to lax your rectal muscles. And misionary goes in at an angle which can be quite uncomfortable. IMO try w/o your partner... let your man go down on you while using a small toy in your anus... slowly increase the size of the toy and when you're using a toy that is somewhat comparable to a penis size have your man do this and after you orgasm (and your anus is a bit stretched and numb) then get on top of him and try it, only allowing the tip to go in and then slowly working your way until you are sitting entire on him, then you can ride him sitting straight up and down slowly. If you start to angle your body towards his face it may hurt but after a while it stops. You can work your way up from there if you'd like.