This is a question that can not have a simple answer, adapted to everyone. Paradoxically, it is asked only when it comes to having himself the role of the low. Now, to justify a refusal of penetration, the answers given by the Member spoke several reasons, more or less formal.
Certainly, given the role of the ass in removing remains of digestion, it is normal to fear that this is reflected in terms of penetration. But when we have a sexual relationship with a partner, we have a degree of privacy that makes an incident can be an opportunity to incorporate gestures cleaning areas that may be soiled, gestures that will confirm this privacy, when each clean the body of the other.
What's funny is that the "dirty" side is not mentioned for blowjobs. However, the penis is used to remove the liquid also from digestion. But that does not bother to take it in her mouth!
There is also that by the parallel with hetero sexuality is the woman who is penetrated. So a man who allows himself to be penetrated by another man is effeminate by that acceptance. In fact, words "top" and "bottom" well underline this concept of domination of one over the other. Couples where both are versatile, does not pose the question because they change roles and the pleasures alternating positions.
Now I have left for last, something that is important and for that, I will discuss my personal experience.
I was married and I had little homosexual experience. When I was forty years old, I met a man with whom we exchanged hugs and blowjobs. I felt great in his arms and one day, I began to feel a desire to go further with him. I felt an excitement in my anus that his penis might satisfy. I have expressed to him and he was glad to hear because he wanted to take me too. So I left him free access to my body by putting my 4 legged on the bed to facilitate access. I was very comfortable and very confident, when he started to tickle me rondelleavec his fingers. Using the gel, slowly he started to penetrate me, one, then two, then three fingers opened and gradually expanded my sphincter to prepare. He was behind me and I let him do it, my final desire to host his penis was always great. When he felt that the move was sufficient, he tried to substitute his penis in her fingers. But when his penis touched my anus, I had a sudden my sphincter contraction which resulted in great pain that prevented his penis to enter where his fingers came and went a while ago. We waited a while and then we try to repeat the experience 2 or 3 times. But each time, there was pain at the time he would get his penis into the open passage through his fingers.
Later, trying to understand why I had not been able to complete the physical relationship with my boyfriend, I think it's my subconscious that played me this trick. My body was ok to be penetrated as it left the fingers move. But my subconscious objected to me to become a "motherfucker" if the penis anus had crossed the threshold and settled entirely within my body. I would not have been a virgin! Later, I was deflowered, but paradoxically, I forgot what circumstances. But I know I must have a certain degree of agreement with my partner, it's not just any penis that can pass.