My 'theory' is that Elia was in on this with Rhagar and DarkStar. When it became obvious that she could not provide the Prince Who Was Promised Rhagar must have questioned what was being done wrong. As the Series is called "A Song of Ice and Fire" and Rhagar himself is the fire and by all accounts, it appears that Elia would be as well, none of their offspring could be the chosen one. But after seeing Lyanna at the joust he probably realized that she was missing element. I believe that they slowly brought her into the fold and sold her on their beliefs. However, to truly clear an easier path the child would have to be legitimate, and yes being a 2nd wife would do it. However, in almost all cases a 2nd wife's child/children are in succession after the first. An issue, if this baby is destined to lead. Annulling the marriage removes the siblings from succession and opens the path. I think he respected and maybe even loved Elia, and if she was a part of this then he would have told her. In fact, he would have had to, he and Lyanna would have to be married at the time of conceiving so that there are no doubts. That, coupled with the 9 months for baby time would leave us a fair bit of time that they were married and Elia said nothing. If she felt slighted it would have been known, as nothing was said she must have agreed with it. I believe in some of the supplemental stories from series that Rhagar was slowly trying to displace his nutty father if this had been successful and he had publicly let it be known what he had done and both women agreed with it publicly, as pissed as their families may have been, nothing could have been done. Also, had this plan worked and Rhagar not killed who is to say that he wouldn't have remarried Elia afterward, as a 2nd wife.