A taboo, for most people? Of course, when you first realize that you are attracted or seriously interested in the best friend of your ex-boyfriend, you may feel slightly weird about it. Do you go after the guy, and risk pissing the ex-boyfriend off? Do you let him get away and then always wonder what could have been?
It is quite a pickle. Now that you know that you really want to do this, you need to realistically assess the situation. Think back — how did your relationship with your ex end? Was it on friendly terms or did you have an ugly breakup? Do you still talk to each other regularly or have you ceased all contact? This information is important, because it makes the difference between needing to tell him about your new-found interest in his best friend or not.
Another thing — how serious was the relationship with your ex, to begin with? Were you in a serious, long-term relationship which you agonizingly decided to end, or was it just a short stint during which you realized you were just not that into each other and mutually called it quits? If things were serious, however, you need to watch out, because he will most likely be a little hurt, or at least, confused. This brings me to my next point: This question is important for a number of reasons: It helps you assess whether or not you would be compatible — were you friendly?
Did you have stuff in common? Did you get along well? This is the perfect opportunity to look at it objectively and decide, based on past experience, if you make sense together and if you actually like him, as a person. It might lead to some trouble with the ex — for example, he might think that the two of you fancied each other the whole time or that you were together behind his back, cheating on him, etc. So if this was an accusation that has come up before from his part, maybe jumping on his best friend is not the best idea, especially if the break-up is relatively new.
That might spark feelings of intense jealousy and even physically aggressive reactions, particularly towards your new beau. Say you have established contact, he knows you like him and he has given signs that he is also interested in you. Preferably, the two of you should tell him together that you want to date. Keep in mind that you might both you and the best friend lose your ex as a friend. Sure, he could take it well and have no problem with the two of you dating.
Or he might feel really betrayed and stop speaking to either of you altogether. You took a chance for love and this is the consequence you have to accept. Even if you do get the green light from the ex-boyfriend, proceed with caution. You have been warned.