You're completely normal, I assure you. People have different triggers for sexual arousal, and I have a fair feeling that most people can't explain why those triggers exist. They just do.
In regards to your never being in a relationship before, do you think sharing your fetish is required to be in a relationship? Your fetish isn't something you have to share with your partner since, from my perspective, a relationship is built mostly on companionship, trust, and commitment. As @jespah mentioned, the sex comes later (unless you're searching for a sexual relationship).
It's okay to feel weird about your fetish. Similar to you, I feel abashed by my obsession with women's feet. The thing is, no one's in your head. No one will look at you and randomly know that you like thongs and lingerie. If you really want to share your fetish, if your partner's wearing a thong and the situation calls for it, you could simply say "I like the way you look in a thong." In that way, you're not necessarily saying you're madly obsessed with thongs as much as complimenting how they look in it.
But again, sex typically comes later. If you start to date someone who you know respects you and cares about your feelings, then you'd have a more open space to share your fetish. If you both comfortably build the foundation of being sexual with one another first, then you could start to share that piece of yourself. Until then, you don't really need to divulge your dark secret