Are there any cruse lines that give discount to SM to use attract them to single cruses?
Actually, there are a couple options!
>> 1. Some of the "luxury" lines still have "gentleman host" programs in which single men can cruise for free (though you might have to pay the cost of travel to and from the ship and certain taxes). To qualify, you need to be able to dance -- ballroom style (IIRC, they require that one know at least ten steps in each of eight or ten ballroom dances, and that one be able to lead proficiently). Still, you can't beat a discount of 100% of the cruise fare on lines like Crystal and Seaborne! Get your dancing shoes off, go see Arthur Murray for some dance lessons, and enjoy!
>> 2. Most cruise lines have programs in which group organizers can receive free cabins (typically one free cabin for each ten paid cabins). Round up your friends, sell them on the idea of going as a group, and you cruise for free! (You might also be able to work out a deal with a local travel agency to lead groups assembled by the agency, as the agency's on-board representative, in which case you don't have to do the organizing -- but you do have to resolve any problems that arise during the trip.)
Regarding the statement that gentlemen dance hosts cruise free, when I researched my Cruisemates' article, "Dapper Dancers Wanted, Men Only" I found that the agencies placing men for these positions charge a placement fee. The amount charged per night of the cruise varies.