This is a combination of two phrases related to the porn genre: "straight porn" and "gay men." In this context, "str8-porn" refers to heterosexual content where the performers are engaging in sex acts with members of the opposite sex. Meanwhile, "4-gay-men" is a reference to gay male porn, which features performers engaging in sex acts with other men.
Putting them together, "str8-porn-4-gay-men" implies a video that has elements of both straight and gay male pornography. It could mean that the video involves heterosexual couples as well as scenes or interactions between men, or it could be a compilation/mashup featuring clips from both genres. However, without more context, it's hard to say for sure what this specific tag might mean in a particular video description