why mirabell is concerned about waitwell's marriage?

She is worried about his marriage because in pretending to be Lord Rowland, even though he's pretending.......... problems (huge problems) could occur with the knowledge that he is not only acting as another but that he's really married as well.

Because Waitwell needs to be married if Mirabell's plot against Lady Wishfort is to be successful. Mirabell needs to get back into Lady Wishfort's good graces (he offended her when she thought he was flirting with her and then her turned her down) so that she will approve of him and let him marry her neice Millamant. To do this Mirabell planned to dupe Wishfort by having his servant Waitwell and Waitwell's wife Foible (Lady Wishfort's maid) help set up a false betrothal between Mirabell's fictional uncle (Sir Rowland, actually Waitwell in disguise) and Lady Wishfort, who very much wants to get married again. Waitwell needs to be married so that the betrothal between him and Lady Wishfort is not valid since you cant be legally married to more than one person at a time. Mirabell's plan is to swoop in at the last moment and expose Sir Rowland as a servant and already married so that Lady Wishfort will be grateful and allow him to marry Millamant. This of course doesn't work out in the end because of the schemes of Mr. Fainall and his mistress Marwood who are working against the main characters (Mirabell Millamant and Mrs. Fainall)