I feel like this is slut behavior, but that is definitely not a bad thing. Being a slut, I feel, is broad. If you reserve yourself for friends or hookup with anyone who's willing, you're still enjoying sex freely and openly; which is what I would consider being a slut. There's nothing wrong with it, just keep doing what you're doing and use slut as a word of power for yourself!
I offer this even to friends who are married with women who don't like anal playing. They fuck me so good and care about me at the same time.
Some couples give me the same opportunity, they let me playing botton with man and toping the lady. That's heaven on earth for me.
Certainly not a slut; instead, it makes me want to be friends with you. There are many valid reasons for having a friends-with-benefits situation. It appears to me that you're providing pleasure to your friends, while also fulfilling your own desires responsibly and with mutual consent.
I am acquainted with a truly affectionate couple who have been deeply devoted to one another for many years. Unfortunately, due to certain medical circumstances, one partner has lost interest in sexual activity. From time to time, with full knowledge and consent from both partners, I engage with the sexually active partner in intimate encounters to provide some enjoyment and relief to us both. They perceive this as a responsible way of assisting a friend, much like your gesture of offering your hole to friends.