Your opening line is, for want of a better word, a “flirt”.
When you first meet new people, their initial impression of you will be based 55% on your body-language, 38% on your style of speaking and only 7% on what you actually say. To be successful, you want an ordinary line delivered with the correct body language, voice inflections and timing.
Your eyes are probably your most important flirting tool. Eye contact – looking directly into the eyes of another person – is such a powerful, emotionally loaded act of communication that we normally restrict it to very brief glances. Even from across a crowded room at a party, you can signal your interest in someone merely by making eye contact and attempting to hold your target's gaze for more than one second (not too much more, though, or you will seem threatening). The distance you keep from the other person when flirting is important, because it will affect his or her impression of you, and the quality of your interaction. When you first approach an attractive stranger, having established at least an indication of mutual interest through eye contact, try to make eye contact again at about 4ft away, before moving any closer. If your target is from a Mediterranean or Latin American country (known as the 'contact cultures'), he or she may be comfortable with closer distances than a British or Northern European person. Smile and don't fold your arms.
The best 'openers' are, quite simply, those which can easily be recognized as 'openers' – as attempts to start a conversation. The traditional British comment on the weather ("Nice day, isn't it?" or "Doesn't feel much like summer, eh?", etc.) will do just fine. Saying "Lovely day, isn't it?" (or a rainy-day equivalent) is the British way of saying "I'd like to talk to you; will you talk to me?" There is a big difference between an interrogative comment such as "Terrible weather, eh?" and a direct, open question such as "What do you think of this weather?". Remember that open questions begin with one of the following words: Who, What, When, Where, How, Why. Negativity, for example, is real turn-off. Compliments, on the other hand, are almost universally welcomed, and do not have to be witty or original. Use humor with care.

Say Hi. Guys don't usually get approached by girls that it would start a conversation immediately. I wouldn't bring up the marriage thing for a while, you will scare the guys away. If you are too shy to say Hi\Hello, just make eye contact and smile. Good luck.

It just depends on your style and way you deliver it. Sometimes, being straight forward works. For example, if I see a girl. I'd tell her straight up, hey i think you're absolutely gorgeous and yet sophisticated which is a difficult combination to look for in a girl, i'd love you take you out for dinner so that I get to know you better. I dont like all the corny jokes and lines to start with, so i'm gonna be honest with you, So, would you like to go out with me for dinner as friends, which is basically the first step for a relationship?

I would ask what his intentions in the relationship are and if he is interested in a relationship or just a good time.

I'll answer from the male perspective.

The best way to approach someone is to have a smile and just say hi. You also need to start the conversation with a reason why you're talking to them in the first place. A reason such as "I think you're hot" isn't a really good reason.

I prefer a more indirect opener based on what you've observed about the current situation. You can then open with a question and then have her respond. Once she responds with something positive, you can then take it from there.

Let's say you're at a party. If you see someone you like, you can approach them and make a comment about how great the house/bar is or ask her about what she's drinking. Showing an interest in her, instead of just looking at her physical attributes is the way to go.