Shame; that makes it difficult for you. I think you have 3 choices:

1. Come up with some fictitious reason why women can never touch the back of your neck.

2. Resign yourself to the fact that you will have to out yourself before they get to that point.

3. Make the best of what you have. By that I mean whatever natural hair you have left, supplemented with hair from a wig which ends at the occipital ridge, plus concealers or maybe a bit of scalp tattooing.

Superficially option 1 is the easiest, but high risk. You're putting your whole credibility on the line, and if the truth comes out you're really screwed. As for option 3, without seeing what you have to work with, I don't know how feasible that is.