Oh boy. Tell you what, there's a lot of weird fiddle faddle out there surrounding bisexual identities. Some people say true bisexuality doesn't exist, some say bi people are just horndogs looking for an excuse to cheat, some say the very term is offensive because it reinforces an essentialist view of gender.
I can get on board with the last one, but otherwise, there are a lot of people, gay and straight, who need to chill about people attracted to both (all?) genders. My feeling is, sexual attraction is fluid. I've known dyed-in-the-wool straights who have fallen for someone of their own gender, and gayest-of-the-gays who have loved someone of the opposite gender. Shit's weird, and you never know who is going to push your buttons down the line.
What is, let's say, less fluid, in my opinion, is the desire/ability for monogamy. It's not like if you are attracted to more than one gender you get a "pass" to fuck other people while you're in a relationship, because nobody needs a pass to do that.
The ethics of what you are "allowed" or "not allowed" to do are fully determined by you and your partner. Like, people who are only attracted to women are still attracted to more than one woman, but if they are in a monogamous thing, then they don't fuck other women.
Same goes for people attracted to people of multiple genders. I know of plenty of bi (pan?) people who are in monogamous relationships. It's not like they sit down and make some pro/con list of pussy vs. dick—generally you fall in love with a person, and are hot for the way they fuck you. What's in their pants and the methods by which they fuck you are secondary.
On the other hand, of course, there are plenty of people, gay, straight, bi, and other, who don't/can't/don't want to do monogamy. Which is awesome. And that can take the form of various committed relationships, or not. The important thing is that everyone is sure their partners are cool with what they're up to.
I feel like there's this thing where monogamists view poly people's open existence as an accusation that monogamy sucks, or their sex life is boring. That's not it. Everyone's wired differently. If we're all just open about what we want and need, we can all get laid and be happy.
So! Long-winded answer: the question you should be thinking about when considering getting serious with your sorta-date is not whether it's worth trying to date a bi guy, but whether this particular one is someone you want to be in a serious relationship with, and whether he would be interested in your definition of serious. How do you know? Same way you ever know anything about relationships: time, awkward conversations, and hours of hashing it out with your friends over drinks.