What are the benefits of having a bisexual girlfriend and what is the downside?

You will have to have dinner with Smelliot Page at certain point. 

It's really no different either way with having a heterosexual girlfriend.  Some think a bisexual girlfriend may cheat on them with someone who is a different sex from one's own (guy's girlfriend cheating with another girl, girl's girlfriend cheating with a guy), but that's no different than the risk of a heterosexual girlfriend cheating with another guy or even experimenting with a girl.  Some think having a bisexual girlfriend could lead to a 3-way with another girl, but having a heterosexual girlfriend could also lead to a 3-way with another girl or even another guy.  Some think it could lead to a polyamorous relationship with a third person, but the same could happen with a heterosexual girlfriend, and in either case it would require all involved being polyamorous.

So really, the benefits and downsides are no different from having a heterosexual girlfriend (or a homosexual girlfriend, in the case of lesbian couples).

The benefits, you can have 3some

The downside, that won’t be ok for you to see her have sex with another person if you are jealous

No different to having a straight girlfriend you perv