Just looking at the prevalence of subject headings in WorldCat
| transsexual | 172 |
| transsexualism | 2722 |
| transsexuality | 31 |
| transsexuals | 1908 |
| - |
| transexual | 11 |
| transexualism | 12 |
| transexuality | 3 |
| transexuals | 27 |
As far as I'm concerned, single 'S' is a typo.
Maybe we should spell sex "sexx" and give it a different meaning too.
Renate