While I can appreciate the concern you are having I think you are putting the emphasis on the wrong question. In other words, I think you are using a hypothetical future girlfriend, and what you want to hide about yourself as a way of avoiding a more direct way of dealing with yourself. The question isn’t what you are going to tell your imaginary girlfriend about your sexual preferences in the past and present, but rather what you are going to do about dealing with this reality in you.
You specifically state that you have had same sex attraction to men, and that you have past relations with men. You do not say you’ve had a relationship with a woman and this leaves what you are saying as only a hypothetical. The scenario you’ve painted is that when the woman you are with finds out that you have been lying to her about your true nature (through omission) she will be upset. In other words, when she finds out you’ve betrayed her with this secret she will ruin your relationship with your family by telling them. You’d want her to keep the secret that you aren’t attracted to her and for her not to be hurt by this. This isn’t fair. Your girlfriend or wife isn’t going to be in an authentic relationship with someone who can accept himself. The “secret” you talk about keeping sounds more like something you are not willing to admit.
I would highly recommend individual therapy with a counselor who specializes in sex-related issues. This is your issue to sort through, not something to lie about to someone else so you don’t have to continue lying to yourself.
At the core is your belief that what you are feeling is inappropriate. You say: “…I know it’s wrong to do certain things with other guys…” This is what is driving you to fabricate a girlfriend and what you will tell her. Working with a skilled therapist to sort through this issue will be more helpful than trying to figure out if lying will be better.
Wishing you patience and peace,
Proof Positive Blog @ PsychCentral