Your partner is not an excuse. The same way I wont accept "I was drunk" as an excuse for bad behaviour, I refuse to accept "My partner made me late" as a valid excuse for being late.
Punctuality lately seems to be something you should be grateful for, rather than a fair expectation of good behaviour and decent etiquette. I almost wonder why I bother to set a time to meet with people anymore, as "meet at 8 o'clock" seems to mean "meet whenever you feel like after 8 o'clock".
The reason lack of punctuality is rude, the reasons it's highly insulting, is that you're telling the person you are about to meet that they are unimportant, that they are not worth consideration, and that their time is less important than your time.
When I am due to meet someone at say, 8 o'clock at say Piccadilly Circus I go into reverse planning mode. So Piccadilly to Waterloo is 15 mins, 5 mins waiting for a tube, 5 mins getting to the tube, 15 mins train, 5 mins waiting for the train, 5 mins getting to the train. I then know I need to leave my flat at a minimum 50 mins before 8. Mentally I'll bank on being ready some time around just under an hour before. As I get closer to needing to get ready I'll look up train times. I see their is a train at 19:15 and 19:08. I'll then leave my flat to make the 19:08, so I have a contingency of the next train should I for some reason miss the first train.