First of all, your job is not depending on whether you sleep with your boss or not. Your job depends on how you do your work at your workplace. So concentrate on your work and stay assured.
Although, your question is not about how to avoid boss, still, if you are not comfortable with your boss, just tell him that what happened once, you feel guilty about it and don't want to do it again. If you like him and the luxurious life he is providing then you can continue...
However, please do not tell your boyfriend about you sleeping with your boss, if you have future plans with your BF. He will certainly not like the casual decision you made by sleeping with your boss.
If you seriously want your BF, don't go with your boss again and forget that one incident for ever. In worst case you may have to find another job.
Choice is yours.