Hello Peter, welcome to the ANZLF.
Matt's suggestion is good, put a bowl/cup/whatever up the leg to catch the fluid then dab and dredge with meths...a lateral alternative would be to arrange a romantic 'thankyou for the pants darling' dinner at home for the betrothed. You wear the new pants but of course being that it is a romantic occasion the lights are dimmed so she does not see the stain.
With the candle flickering and the scene now set you simply bide your time for 'that' moment, preferably when your glass is near empty and the bottle done and dusted, then you lean forward across the table locking your gaze into her eyes and tell her how completely mesmerising she is tonight. Hold the gaze for a second or two and then withdraw with a contented sigh, and then in that moment of love struck clumsiness, you knock your red wine from the table into your lap.....You then stand up quickly and in your most distressed voice you shout "My pants!!! My pants!!! My beautiful new pants!!! Next you tear off your shirt to use it to mop the pants and in the process reveal your naked chest..then you both head of to the bedroom for a shag and a fag and the pants just become an unfortunate victim of passion...I do this all the time and its worked twice so far, I have two kids to prove it.
Cheers
Kim
I didn't have the heart (or the intestinal fortitude) to tell her. 
