Here's what Lee Lehman says:
Astrologer Lee Lehman has an excellent series of CD's to help you learn horary astrology. I am learning from her CD on relationship horaries right now.
She uses the 5th house for lovers, and the 7th for live-in or marriage relationships.
It makes sense to me to distinguish between the two this way, because you might well be dating someone you have no intention of marrying, so the nature of your relationship would not be the same.
Try both approaches and see what works. I have found Lee's approach works.