I never really think about "people" when I go solo per say. Like yes, I sometimes think about so and so's huge cock and that I time I did x with so and so, but I am not actually fantasizing about being with them. I'm just reliving the pleasure I felt in that moment when I think about it, not the actual person or my feelings towards them.
Like I'm bi, so if I am craving some dick I might fantasize about the last time I gave head and how awesome it was, so in my mind I will picture that person's dick and relive the experience as my right hand takes me to paradise. Or, if I am feeling a straight pull in my bisexuality, I might fanaticize about the last time my FWB gave me head or rode me.
The point I am making is, these things I think about while going solo and beating it are not fantasies with actual people. They are just mental images of past experiences that I had, that help arouse me based on my sexual "craving" in the moment. The people I think about during sex is regulated solely to their sexual organs and they are faceless, nameless, otherwise. Some of my favorite porn is glory-hole based because I seldom care for anything on the other side of the wall, I just want to suck that big thick meaty boi meat or watch a hot girl bend over at the glory hole and give some lucky guy a memory that will last him a lifetime. You literally cant fantasize about a person if you can't see the other person on the wall. So for me, its more viscerally connected to our sexual organs, not the person they are connected to..
The point I am making is, for me, often porn or memories I use when getting myself off are just visual stand-ins for the act I am craving in that moment. I am not thinking about so and so's dick, I am just think about that time I had a nice dick and want one right now and get off to the idea of having a nice dick, so I think about a nice one I had. And as a bi person, its the same with straight sex. "Man I really could use some pussy right now", so I sit back, relax, put my cock in my right hand and just imagine that time I I got laid and a girl rode me senseless.
Like its not a fantasy about the person, it's just a mental stand-in for what I want in that moment, but don't have, so my right hand does the pleasure for me while my mind fills in the blanks. So to answer your question, not I don't feel guilty because I don't fantasize about other people when I go solo, I just fantasize about the act I am craving and then I visualize it either in my mind, or with porn.