I still think it's highly plausible, I mean, Ned was always the one to put honour and duty above his own personal desires, he had nightmares of Rhaegar's children dying, what chance has he got forcing a woman to have a bastard? Plus the timing would be horribly off too, since it would take 9 months for Jon to grow, if Ned carried the supposed woman until Jon was born, too many people would have known, whereas in the case of R+L=J, the 9 months that Jon would grow was the period between Lyanna and Rhaegar going to Dorne, Ned and Robert waging war, Robert winning the Iron Throne and Ned going to Lyanna at Dorne, plenty of time for Jon to grow. Plus that promise Lyanna made Ned promise was fishy, I mean, promise her what? Look after Robert? Unlikely as Ned didn't think of Lyanna when Robert died except with the parallels of dying promises. So yeah, there's more things to prove R+L=J than there is to disprove it