This will echo some folks above, but my two cents...

I am a 51 year old male who love giving and receiving anal (straight, with ladies) and I enjoy having my anus stretched...

First of all, don't stick you dick in someone's ass unless you LOVE to have it done to yourself. That is the only way you can help a beginner cherish the first experience. The first time you have something in your ass it feels exactly like you are passing a bowel movement, even if you just had an enema. This is mental barrier you have to deal with

Start with JUST enemas for a month or so, so that your lady friend will learn to trust you. This will help you both mature up to dealing with fecal matter also. Then perhaps some rimming for a month or so, then a SINGLE finger while you massage her body or performing oral sex on her. Go slow, slow, slow; even if that means taking 6 months or a year....

ALL penetration has to be with LOTS of lube. If a finger, lube the finger completely, then gently beging to lube the anus, slowly with gentle circles/soft strokes that parallel the anal cleft and that gradually lube up the passage, take a break and then lube some more, so on and so forth. This action will massage and relax the external and internal sphincters. The INTERNAL sphincter is the gate you have to treat with kid gloves. Before you know it, your finger will seem to "fall" into the anus/rectum.

Never put a drop of lube on your finger tip and jam it into someone else's anus. This is not sex. This is assault.

Anything larger than a finger... let your lady friend choose the control method, which is to say that she controls the penetration, not you.

Anal should NEVER hurt!! I know full well that some people blurt out "It hurts." like a bad habit, even if you haven't really begun yet. Never the less, you must respect anything signal that can be interpreted by a Judge as No.

Another idea is to date lots of women until you find a few who initiate anal sex and go from there.

Going back to the beginning, do your own ass first and do it often. If you can't grab medium size cucumber and have it your ass is just a few minutes (assuming your are poop free) and ENJOY it, you have little business doing it to someone else. It has nothing to do with being gay or straight. The anus is an rich erogenous zone for all men and women.