Have you tried masturbating before you meditate?
Sexual thoughts are a hindrance for sure, but it would be quite ironic if the hindrance to your meditation practice was making you feel bad about yourself. If you don't currently have the mental stability to experience them without reaction, it might be better to just work around the problem for the time being.
Group meditation would probably help, too. If you end up having sex with another participant, it's not that big a deal. Just keep plugging away. At the meditation, that is.
It's a good thing the Buddha is not a vengeful God. He would probably strike me down for saying this.
If you really want to tackle this head-on, I would cultivate awareness of compassion or joy. For compassion, you'll probably find there's a contemplation based on the crucifixion you could use. There must be something in Christian contemplative practices for joy, too. Try compassion first, then joy. Joy for abstaining, not joy in getting off.
But in the context of my own practice compassion and joy are at the least intermediate practices because they require quite stable attention, so this may be a difficult and untimely choice.