Seems the less complicated choice for the lady is the sponsor. I mean age is not even an issue and the support is > probably. While with the married BF, time, finances and the affected party will be the difficult metrics.
The question is if the sponsor is the easier choice then why the turn down on the proposal. My opinion, true emotion is resilient it will thrive no matter how difficult the situation is. Only the lady can answer your question and she probably needs more time.