Since I didn't see anyone else mentioning it so far in this thread, I thought I would point out that often a fraternity jewelry maker has special "sweetheart" pins and jewelry specially made for this purpose so that you don't have to give up your own member pin. You might not see it advertised on their main website or ads, but if you inquire, you may find out about it.
www.BurrPatt.com is the maker of the jewelry for my husband's fraternity and the website doesn't have the sweetheart stuff listed, but my hubby called and asked them about it and they sent out a whole list of items that were available, including a sweetheart pin and special sweetheart lavaliere.
So men, investigate your options before you give away the pin you were initiated with!