Taking the train from FRA to Trier, and thought wife and I would stay onvernight in one town along the way. Any suggestions on a romantic hotel and town to stay at in May?
Bacharach would be a good choice. Good list of hotels on the town website www.bacharach.de
I've stayed in Bacharach a couple of times. The following website has a number of great places to stay there: http://www.bensbauernhof.com/accommodationsrhinemosel.html
There are lots of attractive towns along the Rhein, and also the Moselle. I would suggest that you first consider whether there is anything you really want to see (just for example, Deutsches Eck and the fortress/cablecar in Koblenz, or Burg Eltz near Mosel-Kern - but I'm not advocating any particular destination!) What I mean is that you might overlook something that really interests you (like the trail of Hildegarde von Bingen, or the invention of bentwood furniture in Boppard, or the old resort hotels in Andernach, or the appointment-only wineries in Bernkastel-Keus) if you pick a town first, and look up the attractions later. We'll remember a stroll through the vineyards of Traben and an easy scramble up the monument hill in Trarbach, both with great views. But most of these towns are half-old and half-modern. So some of the romance is hidden. For example, Bingen is very historic, but they took the opportunity of the 2008 Garden Show to redevelop their riverfront. Also, think about the towns where you have to change trains anyway. You didn't mention a river trip, which is a must for most people staying on the Rhein. So you might be picking a town to get off at, and another to ride downstream ("back") to spend the night in.