We wouldn't be the people we are if God didn't make us this way. I can't say I'm bi but I've had sexual relationships with many women and though it is not in my nature to enjoy returning the favours I enjoy getting from them I would never look at myself as sinning for partaking in such indulgences. Nor would I ever look at any persons lifestyle choices as sins or need for spiritual guilt. It is not my place, yours, or even hers to determine what is a sin or sinful. Jesus himself took company with many people that others considered sinners, but Jesus called them friends. So I believe it best for her to take comfort knowing though she is capable of condemning herself that condemnation doesn't automatically turn into a holy one.
Perhaps I'm wrong but the vibe I get from your post is that you tend to take the traditional biblical role of being the head of your home and your wife the heart. If that's the case then perhaps she is seeking first more security in how the head of her household views this subject in regards to the Lord instead of your bedroom. I don't mean this offensively at all, but I do know as a person who has struggled with some things I often find myself seeking just that. Perhaps if you have a purely spiritual conversation about what constitutes feelings of spiritual guilt in your household then later after reflecting on that she will find a way to come past the guilt she's come to associate with exploring this side of herself.