How to make it happen
You have to take chances, but not risks. First, I agree about the photo. That and your sweet first post might be the first step.
But assuming not, go places where there are couples, and have already written a slip of paper with your name and email, and something sweet but not overtly sexy...just add, "loved meeting you."
If you see a couple that seems loving and into each other, find a way to talk to the female (or dom if it's lesbian). Say how much you admire their loving relationship, and say how sexy she looks when she's with him, but not as a come-on. When they are back, if it's comfortable for you to join them (they'll give clear signals if they are uncomfortable), tell enough about yourself to indicate that you are bi and single. Don't make a big deal about sex. Covertly hand the slip to the woman as you leave.
This is overt enough to all concerned: you've not tried to hit on her man;
you are attracted to them not as lovers, but as a loving couple. The next move, if any, is up to her.
Naturally you only do this with with couples who seem comfortable in their relationship.
Be prepared for failure. Not many couples are affectionate in public. This is a slow way. You can work out other slow ways (after a show, seek out the woman and ask if there is a nice place to go for a drink or coffee), but they all require you to put yourself out there.
Plan B. Craigslist. Yes, I know, I know. But real people post there, not just creeps. Check out this genuine account by a woman who wants to give her husband a threesome for his birthday.
http://www.marieclaire.com/sex-love/relationship-issues/threesome-sex-menage-a-trois-planning-2. Of course, in this case you will be the one posting.
If it happens, make it as relaxed as possible. Only afterward, if you liked it, be affectionate and loving. Most couples are afraid of all the baggage that comes with inviting another person into the relationship. Friendship is the only way to make that happen. So if you get this far,
be a friend, not just a conspirator, because if you want a poly relationship, it must be among people who genuinely care for each other.