Common regardless. Sexually transmitted diseases are common in any individual who is not in a strictly monogomus relationship, has had more than one sexual partner, and has unprotected sexual contact regardless of sexual orientation.
No. STDs are most common in people who have unprotected sex with multiple partners. Each STD has its own predilection for sub-segments of the population and there are some differences in susceptibility between males and females. If you are careful, and use protection, then man or woman, gay or straight, you never have to worry about STDs.
YES. Stds can result from sexual contact whether it is homosexual or heterosexual. The best protection is to always use barrier protection such as condoms.