It doesn't matter what your sexual orientation is. You said no. The next time he asks, say "No, I'm in a relationship. Please don't ask again." If he does, then it's harassment.
As it sits right now, you and I can be friends and thats just about ALL!. If you ask me out one more time however I wont even be your friend. I'm really not interested in dating you now, or in the future. thank you anyway.
tell him that you may value him as a friend but absolutely nothing else at the moment as you have an other commitment
saying no gently is enough. if not then say no more firmly. it's just no one's business on why you aren't interested. consider it a compliment but then if haresssing to you. kick his ***.
You don't need to tell him anything. And you should be open about your sexuality when and if you want to be - not just so he gets the message. Tell him that he is not your type, and he can interpret that however he wants and remind him that his persistence is bordering on harrassment, his attention is unwanted and innappropriate, and if it continues you are going to report him. Also, tell someone else, or say it in front of someone so you have witnesses. What an egotistical pig - Im surprised you haven't kicked him in the knackers yet :)
You dont need to tell him and you dont have to tell him - unless you want to use him as a way of coming out to your co workers<br />
BUT if that is your plan - be carefull it may not go well. Never let somebody else tell people what you are, because they wont know it all and they wont do it with the care that you would do it.
Possibly the most effective answer is that you do not get involved in social relationships with people you work with because should the relationship go badly then it would cause problems on the job. <br />
In fact that is why a lot of companies will not even hire people in the same family and in some workplaces if two people get into a serious relationship then one of them has to find alternate employment.<br />
As for telling anyone your sexual orientation, that is up to you but may be better not to within the workplace. That could result in him tryign even harder to date you and/ or make things uncomfortable. <br />
As for saying you are in a relationship, well, unless it is true you, might not want to especially since he may discover you aren't and then get ticked off for being lied to.<br />
Basically, he should accept the 'no' and move on. If 'no' and an explanation about not dating coworkers doesn't work then politely inform him that you might consider further requests for a date to be a form of harassment.
When I get asked out I say "I'm not really interested in guys atm". Men usually assume that means I'm gay, but it's ambiguous enough that I can correct them at a later date if I need to.