Fluffbags, on the first page (I don't want to quote the whole post), makes some very sound and common sense points as always, so thank you for that.
This is a multifarious issue and the reasons manyfold. As others have said, some men have a very macho self-image and don't want to be seen to be doing anything that could be, even in the remotest contrivance, construed as being homosexual.
Thank God this practice is getting less now, but I have seen so called doctors and experts appearing on television condemming any form of anal activity other than it's natural function as dangerous or prejudicial to health and biggot-minded comics criticising all homosexual activity and making hurtful jokes about it. The problem is this sticks in the 'macho' mind and reinforces the attitudes that some men were raised with.
Whilst it may be true you have to be careful what you use on/up the anus (we don't want any embarrassing questions at the A and E department), as long as the toy is properly designed and thought out from reputable supplier, then an awful lot of fun can be had from anal and prostate stimulation.
As Fluffbags said, without directly quoting, a bad experience can also put people off anal. Rushed play, failure to relax or give suffcient time to relax and badly/insufficiently lubricated toys are common causes of men who have tried it, not wanting to try again.
Lack of empathy from a partner can also be a root cause.or macho mates who make jokes about it being 'gay'.
I am straight man (I always thought that was a stupid term anyway) and happily married with great sex life. However, we enjoy pegging, etc and this would have me branded as homosexual in macho-man's eyes or indeed my friend's eyes (not that we discuss our sex-life with anyone as that is our private business and on here we have complete anonymity). Being homosexual, means wanting a loving and/or a sexual relationship with a person of the same sex, not what you desire to do within your own sex life.
It's good that attitudes are changing, but it's too slow. Homosexuality is, thankfully, now more accepted as a normal lifestyle and being called (regardless of orientation) 'gay', hopefully won't have the same impact on people as it once did and will lead to a 'so what?' attitude in society.
I have a maxim in life that says, do whatever makes you happy, as long as you don't hurt, upset or embarass any other person in doing it. Also, and this is covered by the my maxim, keep it legal.