I am a triathlete and cardio whore. Way less then I used to be though, explained below...
I'm out for about 1 hour a day, 5 days a week
Doing:
Intervals
Sprints Uphill
Cycling
Running
Swimming
Mountain Biking
Tennis
Skipping
Ergometer (Rowing Machine)
Ice Hockey / Roller Hockey
I'm very active
Definatley affects muscle mass.
I'm more into the bodybuilding now so I've cut back on my cardio sessions and the length of time I spend on them. I also only do triathlons for fun now and keep it to the short distances.
The past few years I was a competetive triathlete and once spring came I would drop from my winter weight of a muscley 185 all the way down to a scrawny 155 in a matter of 2 - 3 months. Keep in mind my training was 2 -3 hours of intense cardio 6 days a week so that I could be at a highly competetive level.
Then I'd spend the fall/winter trying to get back to where I was. I decided that I needed to make a choice. Doing both was just dumb since they countered each other.
Don't be afraid of cardio unless you're out training for like 2 hours 5-6 days a week. 45 minutes 4 times a week won't have much affect on your muscle mass but it will definatley increase your fat loss!
Cardio also has assloads of health benefits for your heart, lungs and cardiovascular system which will keep you living a longer healthier life which I'm assuming is important to a lot of people.
Just my 4 cents