I'm all for people doing weddings for close friends or family or whatnot. The photographer should know what they're getting into. I'm a big fan of people lying in the bed they've made, although I have to say I have some sympathy with the pros who have some strong opinions about people doing first time weddings without the proper training or gear. It's like telling someone who's only ever driven an automatic, putting them in an F1 car and telling them to drive at 200mph. You're not ready and chances are you're going to crash. But... if it will make the participants happy and get you experience, go for it. Just realize that those pictures with your name attached will follow you far past your ramp up days and you won't always be there to explain "oh, that's when I was learning." Photos tend to take on a life of their own after they leave your creative and distributive control. You have to at every juncture learn to live with that.
I do take some exception to this:
Without producing great pictures and producing them consistently, no amount of customer service is going to mitigate the damage terribly awful pictures will do to that business relationship or future business. It may not be the only important thing, but it is the thing on which all other factors depend. No product. No business.
When it comes to photography and professionalism, the professional must produce results and they must do so while mitigating factors which might otherwise keep them from producing those results. For a head shot photographer, the risk is generally lower because if you have a gear failure, you can redo the shoot (though there will be outliers who are working on very strict guidelines). There's a continuum of risk depending on the type of shooting you do... model shoot for a magazine? Think rented gear, space, time. Wedding and event photography are a hot button issue because the skill requirements to mitigate any potential risk are some of the highest in photography.
Know what you're getting into. Though it may sound like they come out of the woodwork saying 'you're not ready' and it sounding territorial, they're also speaking from experience. It's often to your advantage to read between the lines, even when someone might be being a jerk about it.