Viagra is for erection, not orgasm. This is a common side effect of antidepressants. The way to get around it is to get creative, it may take awhile but find what really turns you on. Everyone's got something.
There can be sexual side effects with SSRIs like Lexapro. I have never taken Viagra, so I would not be able to really share an experience with it. Taking an SSRI, usually does have some side effects and the benefits often have to be weighed with them. You are definitely doing the right thing, in my opinion, by working through this with your doctor.