Oh boy. Well this topic finally got me to crawl out of the shadows...
At the end of the day, this seemingly simple question is a ball of competing issues and stereo types. As a result, it's not going to have a simple, pat answer.
To start with, I'm a man who is sexually attracted to both men and women. I have had sex with both men and women, on a few rare occasions that includes both men
AND women at the same time. In the spirit of full disclosure, I never have what I'd call a long term relationship with a woman. I have been in several multi-year relationships with other men.
However, please note, I'm not labeling myself "bi" or anything else except as "sexual". That is literally my response if someone asks me for my orientation: "sexual", or if I'm feeling snarky "yes, please". The reason for that is labels are part of the problem here. I learned that a long time ago when I was figuring out my own sexuality.
- Men who identified as "gay" would get on my case if I called myself "bi". They insisted that as I had attraction to men, I could only be "gay like them" and I was just afraid of, or hiding from my true identity when I called myself "bi". They couldn't fathom that someone who was sexually attracted to men could also be sexually attracted to women.
- Men who identified as "straight" would get on my case if I called myself "bi". They insisted that I just needed to "grow up" and quit playing around like a boy. I just needed to meet the right women and settle down, maybe have a kid or two. They couldn't fathom that someone sexually attracted to women could see anything sexual in another man.
Neither group understood the simple truth of the Kinsey scale. Human sexuality is a continuous spectrum, not a few fixed points. Where a given man falls on that spectrum may have an impact on whether or not he's going to tend to form relationships with a partner of a particular gender or if he's just having recreational sex with them.
Then come the questions of emotional intimacy. I strongly suspect that this also a continuous spectrum, we just don't have a name (that I'm aware of) for it. We all have the ability to form emotional bonds with both men and women, but we may lean one way or the other to greater or lesser extents. Again, this is going to vary from man to man. On top of that is the issue that emotional intimacy and physical intimacy are always comfortable bedfellows.
There is an old saw about this that I don't exactly remember, but it goes something like "you can't marry the person you want to sleep with, and you don't want to sleep with the person you want to marry". The idea being that we are frequently sexually drawn to sexual partners who are risqué, promiscuous, kinky or what-ever, but social norms also preclude those people are appropriate partners for a LTR. On the other hand partners who are socially acceptable for LTR's frequently get treated like delicate china figurines, handled carefully and great delicacy, and never exposed to things like our raw sexuality for fear of exposing ourselves as some sort of sexual deviant, or of alienating them because we're too crude, or both.
So not only is the scale of emotional intimacy/bonding a variable, but it's variability is also compounded by other inhibitions and quirks about sex, sexuality, and their interaction with emotional intimacy.
On top of all of that you add the scorn of the jilted partner. I don't care who you are, of what your sexuality is, it just sucks to get dumped. As a matter of human response, we always try and make it about something other than the simple facts: "he dumped me because he was just using me" instead of "we weren't a good match", or even the realization that it really was recreational for both of parties, it's just that one wasn't ready for end quite yet.
That all adds up to "no simple" answers. I have known women who feel that all "bi" guys are just using them for a quick thrill and are really gay (but too afraid to admit it.) I have known gay men who feel that all "bi" guys are just playing around and are really straight, but they need sex now, and couldn't find a woman to do it with. And I have known other bi men who have ultimately formed long term relationships, some with men, some with women.
There isn't a pattern or a smoking gun. There is no single truth. All of those things are true in some cases, they are false in others. And then there're all the other possibilities in between that don't lend themselves to easy descriptions. This is also a spectrum. It's continuous from end to end, each individual situation will fall somewhere on that spectrum. At the end of the day, with enough samples, I'd expect the distribution across that spectrum to even. No lumps, no spikes, no nothing, just flat and boring.
<Stepping off the soap box>