So my Boss asked me out 2 months back and it was tough saying no because i find him exceedingly attractive.He so wants to sleep with me and i am finding it hard to keep saying no.He s married .what to do?

All of the above, plus document everything. Tape record him if possible. And keep saying  NO.

Politely remind him that his repeatedly asking you could be construed as "sexual harassment" as well as "creating a hostile workplace" ... Then remind him that he is married, and ask what his wife would do if she found out.

btw - if this is in the US, in some states the wife could sue YOU if you do get involved with him.

Next time he asks you, remind him that he's married....then tell him that once he's divorced, he can ask you out at that time... But until then, tell him to leave you alone.

Find another job !  Do not .. I repeat do not get hung up on this man .. He is a user (obviously)  and sexual harrassment on the job is against the law! You mean nothing but a play thing to him .. Clearly.

He doesn't care for or respect his family,  the law, or you .. so what exactly would you get out of sexual favour for him?  Dont fool yourself into thinking he will suddenly become someone worth getting involved with .. the man has no integrity. 

I get the impression that you've already made your mind up. I hope I'm wrong.