I've read your column (and other sex columns) for years, and one thing I always notice you saying is that all fantasies are OK, and fantasizing about something can't hurt anything. But what if you don't think your fantasy is OK? I'm a lesbian, I have a girlfriend, and we have a pretty satisfying sex life. Even so, I'd understand why if I (or my girlfriend) were fantasizing about other women, or about things that we're just not going to do, like S&M (some of our friends are into it but it's not for us) or threesomes. But I'm not. I'm fantasizing about guys! When she fingers me, I pretend it's a cock. I don't even like cocks! I haven't sex with a guy since I was 16, and I stopped because I didn't like it. So what's going on? I feel really bad about it, like if my girlfriend knew she'd feel betrayed, and also like I'm betraying myself. I'm happy being a dyke. I AM a dyke! So what the hell?
Love, Confused, guilty, still a dyke
Of course you are, dear. You are a dyke and nobody can take that away from you, so no need to be so defensive. We believe you. The question then is, do you believe you? Are you really a dyke? Really? You really think so, feel so, know so? OK then. What are you worrying about?
Right. Your girlfriend. Well yes, it is entirely likely that she would find your fantasy life appalling, especially if, while cluing her in, you emphasized the part about pretending any part of her body is ... one of Those Things. If you do decide to tell her what's going on, you're going to want to rephrase that. Fantasizing that there is a Thing around somewhere and fantasizing that said Thing has replaced your girlfriend are not at all the same thing, and you're going to want to try to spin it in such a way that she hears that you are super-satisfied with her and just happen, also, to fantasize about one of those bad horrible Things that of course she could not possibly have, nor would you would never wish she did have. Are we all clear on that?
You would also want to emphasize that you are not thinking about cheating or answering one of those ads from straight guys looking for the kind of "lesbians" they're used to seeing in porn movies. You're not looking for man, just thinking about a Thing. A Thing completely unconnected to a person. An imaginary Thing.
Your other choice is, obviously, not to tell her. This is actually the way most people go, and despite my officially endorsing relationship glasnost as much as possible, I don't actually believe that you have to tell even your nearest and dearest everything. If everyone did publicly confess every vile thing that had ever crossed their minds anywhere along the sexual response cycle, it might have a salubrious effect on society in general No more shame! Everybody's kind of perverted! but then again, it might just as well make for a lot of really nasty fights and some divorces, and to what end?
I can only think of one reason to tell her, but it's a big one: there is a chance that she will look startled (which will terrify you) and then confess, all in a rush, that she has similar fantasies and was sure you'd freak out if you ever knew, and then you could both laugh and forgive each other and yourselves and live happily ever after. But frankly, I'm still on the side of don't ask, don't tell (and don't quote me).
But how your girlfriend would react is not really the question anyway, I don't think. I think what you really need is to feel OK about it for you. I can't make the fantasies go away (and neither can you). I can't reach through the screen here and therapize you, or hypnotize you and make you repeat "It is OK to fantasize about things I do not want to do" over and over until you believe it. All I can do is tell you that I have heard the same things from lesbian after lesbian.