I gather his son and partner have been living together for 10 years and this "marriage" is only a formal tying of the knot and not a huge Italian wedding.
And bearing in mind the pressure he is under as England manager, if he missed the match you can imagine the headlines:
"Paid 4 million a year, and playing one of the top teams in the world, and he is off to Spain for a family party".
He cant win either way (and I dont mean the match)
Anyway there are LOADS of weekends when his son could have chosen to marry, when Capello would NOT have been managing England (they only play about 12 games a year and most not on Saturday).
Why did he pick a weekend when it was an international break all over the world (these dates are set in the diary years in advance).