Women travelers face no particular discrimination or dangers in Thailand, nor is there anything permitting them entree to Thai culture. Women should, however, be very careful when dealing with monks: Never touch a monk, never hand anything directly to them (should be set on the floor in front of the monk or given to a man who will hand it the them directly) and don't sit next to monks on public transport. Some parts of temples do not allow women, but there will be signs to that effect.
Thais are modest, though the scanty fashion trend in places like Bangkok would tell you otherwise. Women should avoid short-shorts or going topless at beaches. At Muslim temples, quite prevalent in the south, be sure to wear a long skirt or trousers and have shoulders covered.
The only other problem single women face in Thailand is the constant questioning: "Are you married?" Friends tell me it's like everyone is that grandmother who is eager for more grandkids. Thais commonly marry quite young and get right to the business of building a family, and anything short of that is, to Thais, somehow pitiable. Don't take it personally.
Women Welcome Women World Wide (5W) (tel. 203/259-7832 in the U.S.; www.womenwelcomewomen.org.uk) works to foster international friendships by enabling women of different countries to visit one another (men can come along on the trips; they just can't join the club). It's a big, active organization, with more than 3,500 members from all walks of life in some 70 countries.
Check out the website Journeywoman (www.journeywoman.com), a lively travel resource, with "GirlTalk Guides" to destinations like New York, Hong Kong, and Toronto and a free e-mail newsletter; or the travel guide Safety and Security for Women Who Travel by Sheila Swan Laufer and Peter Laufer (Travelers' Tales, Inc.), offering common-sense advice and tips on safe travel.