Yes it can take awhile to show up, I think up to three months, the IgG number rises as it seroconverts.
This is a great forum for any questions you have www.medhelp.org/posts/Herpes/
If you don't end up getting a positive test, most people I know who have a partner with HSV2 gets tested every 6 months and use barriers with them (up to 90% risk reduction
but there are conflicting studies on how much) so if they get it, hopefully it is realized before its passed along to other partners so everybody can make informed decisions.
Now of course most people don't want to start using barriers with an existing serious partner, and are willing to take the risk, as often the actual having it doesn't turn out to be a big deal, barring the immuno-compromised who really need to avoid it, and the awkwardness of discussing it with potential partners. Learning to discuss it in a calm fashion seems to be the most important thing, if you are freaked out and think its a big deal, people are more likely to respond to that energy instead of checking out the statistics and deciding that way.
And there is always the chance M might take a chance on you....but you MAY need to have another successful interaction with somebody else first for M to see that it IS OK, than J was OK with it, that everybody was treated well, and believe that you're not just looking for a sex toy, or to leave J, or whatever other concerns she might have.