Is there a particular reason you should tell them your sexuality either way? "Hi, I'm <insertnamehere> and I'm gay". You do realize that doesn't seem very intelligent, don't you? Not that saying right off the bat that you are straight makes you look any smarter.
Japanese people don't discuss very personal issues with people they aren't close friends with, and I believe it is seen as improper to do so.
Translated from a Danish (of all languages
) book about Japan:
Japanese people are largely friendly and helpful, but as a tourist one shouldn't expect to get close to the local populace. In Japan one doesn't take friendship lightly, they often last throughout life. Visits in private homes are usually restricted to family, close friends and co-workers.
Occasionally one will meet Japanese who are interested in small-talk (I think, the Danish word "Passiar" is completely unlike any Norwegian word I know). The main theme is (of course) Japan, how one feels about the food etc, and of course your own country. The conversation is kept at a polite, light and interesting level. In-depth explanations and very personal opinions aren't considered proper in Japan.
Personal interpretation of an excerpt from the Danish edition of "Turen går til Japan" ("the trip goes to Japan", literally translated), which I got for Christmas for some inexplicable reason. Probably the shelf I filled with manga (and I requested another shelf) had something to do with it.