I guess everyone has their secret fantasties and it's best to look at this that way. I personally wouldn't have any problems carrying on with this - you're an adult and it's not hurting anyone (except possibly your boyfriend). To keep yourself safe, stay anonymous by using a fake name and address, send no photos, especially with your face in and never accept any offers to meet or anything of the kind, though I'm sure you are sensible enouhg to know all of this already.
With regards to your boyfriend, it's down to you whether you tell him what you've been doing or not, and only your own conscience cna decide.
Of course, if you stopped doing it altogether, you wouldn't need to tell you boyfriend, and just see it as an experiment and no more.