ANSWER - Interesting question. My first instinct was to say no but after a bit more thought I'd say it isn't impossible - there are circumstances in which I can imagine such a spell being worked.
The crucial factor to my mind would be how deeply rooted a person's individual sexuality is. Sexuality runs on a spectrum from hard straight to hard gay incorporating all the many possibilities in between. I actually find it hard to get my head round the extremes of both ends. As a dyed in the wool bisexual I can't imagine not fancying everyone! But it is important to respect that some people simply are homo/hetero sexual with not an inkling of interest in exploring the opposite end of the spectrum. Do I think magic would work to change their preference? I suppose it might but depending on what type magic was cast. Certainly you couldn't change the intrinsic nature of a person to the extent that you switch them from straight to gay or gay to straight. You might however be able to plant the suggestion to open their mind to the possibility or at least to get them thinking about it. You may also be able to work magic to make yourself more masculine/feminine and in conjunction with a spell to open the target up to the possibility of exploring an alternative sexual path this might have some effect. I'm sceptical though, spell work to change the basic nature of a person is extremely hard to pull off and with something as personal as sexual preference I don't think you'd have much success.
With the people in the middle of the spectrum this is potentially easier magic to work. Using myself as an example I can see how I could be enticed to prefer one sex over the other. I sit right on the balance so I'd only need a light push where the absolute homo/heterosexual would need a hell of a magical shove. If your target is already open to the idea of a flexible sexuality you could work magic to encourage them to experiment. You could certainly work magic to persuade a bisexual that you were their chosen sexual ideal - this wouldn't be vastly different to the typical love spell.
In the case of your question, the way you word it suggests you believe the person may not be entirely telling the truth about their sexuality anyway. In this case you might be better going down the route of a full disclosure and honesty spell to get them to admit the truth about their nature rather than trying to employ magic to change them.
I'd add the caveat that even if it is possible to influence a person's sexuality it is a pretty immoral thing to do even more so if it is done purely for your own benefit in snaring them as a partner. Taking something as personal as sexuality and twisting it for your own needs is using that person as no more than an object of gratification. It is saying their identity matters less than your pleasure and although no doubt you will find some witches who would condone such a thing - I wouldn't. And as with any form of love spell even if you did form a relationship this way it isn't based on genuine emotion, it would be based on manipulation. I can't see that relationship having much of a future and the insecurity it would bring you of knowing how flimsy are the foundations of your love would destroy any hope you might have of happiness.
So theoretically Yes - should you do it - No.