Should Christians serve Sinners?

A common question I get asked:

Should Christians serve people whom they believe are living a "sinful lifestyle." Behind this question is the fear that by serving cupcakes to sinful people, somehow Christians are selling out and endorsing said sin. Setting aside the tangled web of trying to define what exactly a sinful lifestyle is, I think this is a bizarre question. Just read what Jesus said according to Matthew: 

“You have heard that it was said, ‘You shall love your neighbor and hate your enemy.’ But I say to you, Love your enemies... so that you may be children of your Father in heaven; for he makes his sun rise on the evil and on the good, and sends rain on the righteous and on the unrighteous." Jesus in Matthew 5:38-48

Is the Father, by bringing rain on everyone, endorsing everyone's decisions and lifestyle? 

(hint: of course not.)

If Christians are supposed to be children who are imitating their Father, wouldn't that mean generously blessing all people?

(hint: of course it does.)