When I confronted her about it, she said that our relationship had felt stagnant for a long time, and while she considered us good friends and good parents, the romance was not there. She thought I felt the same way, that we had grown apart.
This is straight from the cheater's handbook. I'm not saying there is no validity to it, I'm just saying be careful about assuming this is the fault of your own relationship issues. She's still a grown woman that should be responsible for her actions. Almost all people, when caught in an affair, use this kind of justification.
It sounds like you guys need a different counselor if his advice was to "force" yourselves to fall in love again. But there is a kernel of truth there that relationships often take work, especially after broken trust. And I hope you are not letting her slide on being responsible for her actions. She could have come to you and honestly discussed your marriage, agreed to take steps to work on it, read books, taken marriage counseling. An affair is a betrayal of trust that is not to be taken lightly.
I am going to also agree with Emm here. Relationships really do need to be on pretty solid footing for the expansion to non-monogamy to work well. I know there have been exceptions to this rule, but more often than not, the added effects of the NRE, all the new emotions involved, the scheduling problems, and the new dymanics can play havoc on even solid marriages, much less ones that have been cracked by an affair. If you can stomach it, read my sordid thread for what can happen when you take on this lifestyle after an affair.
Of course, every situation is different. Mine is complicated by my W's bi-polar disorder and her extreme anger issues, as well as overwhelming guilt at bringing children into the world that she has not bonded with.