Even if there were a mandatory field that said 'sexual preference', unless there was a limited menu of options (and who wants to limit themselves to the boring 3 hetero, homo and bi? I've always liked the idea of pan and omni sexuals though I was confused recently by a reference to asexual on fetlife, I may ask someone who chose this if they really do divide down the middle to reproduce... ) those who preferred not to say could simply write 'mind your own effing business' or similar. Anyway, even if there were limited options, most sites that have this option (and one or two job applications I have seen online recently) have a 'prefer not to say' option. I think in the case of job apps its actually a legal requirement (linked to 'don't ask, don't tell')

Basically, there are always going to be ways around any attempt to force people to reveal their sexuality up to and including lying about it.