Billy, yes, it is possible for an angel of darkness to appear as an angel of light. It is also possible for angels to appear as men for the purpose of communication, as happened frequently in the OT. (No appearances of angels in the NT, though, after the birth of Christ, and I don't know of any reports in history of people being visited by angels, either.)
There is nowhere in Scripture, however, that implies that an angel, even in human appearance would be capable of sexual intercourse.
The only possible explanation for the giants I can come up with (and this is pure conjecture on my part) is that the Bible never tells us what the mark on Cain was. Maybe he became a giant and his DNA was altered to produce giants? No idea...
Other than that, this is just one of those things the Bible is not clear on. Maybe the bit about giants is simply an add-on comment that really has nothing to do with them being the RESULT of the marriages between the sons of God and the daughters of men.