The only "rule" is that the couple cannot appear in a married monogram until actually married. Thus, if you make a monogram with the man's last initial, you can't put the woman's first initial with it until the ceremony is complete. Not even if the couple, coincidentally, start out with the same last name initial.
If Homer Simpson marries Marge Bouvier, then a hSm or mSh monogram would be OK after the ceremony is complete. You would not put it on wedding stationery, etc, in anticipation.
What you CAN do is intertwine the first name initials. Personally, I think that looks really nice, sweet and romantic. When Sarah Ferguson married Prince Andrew, her wedding dress had a long train with an intertwined "S" and "A" embroidered at the end. (Too bad they got divorced later.)
The reason why some monograms are in the bride's name, only, is because in times past, a young girl would monogram linens and then store them in a hope chest. She would do this long before ever becoming engaged, thus, would not know her future married initials. Thus she would monogram her own maiden name initials.
Once she gets married, she would monogram any new linens with her married name initials, or those of her husband.
I remember reading a eighteenth-century novel where houseguests commented on the fact that the couple were still newlyweds because the wife's maiden name initials were monogramed on the bedsheets.
Later: I put "intertwined initials" in Google, and found a really great web page. You might want to have a look. Great inspiration!!