I was honest, and that's why they nearly deported me. The *did* suspect I was going over to get married, but they also suspected I was going to look for employment. At the end of the day, all they have to do is suspect.
If you say why you're really there, to live with your girlfriend and see if you want to get engaged etc, I think you'll have a hard time. That's not what a tourist visa or passport is designed for, as unfair as it is. If you're going over to be with your love, and things are great, what's going to stop you from heading down to the local Registry Office and getting married? Nothing. And the people who will questions you will know that.
At the end of the day, it's a decision you have to make on your own. We're devided here obviously. I'll never think it's a good idea though, just because of what happened to me. I was honest, had the best intentions, was doing nothing wrong... but there was a lot of potential there for me to get around the rules and they don't like giving people that much rope.
That all being said - if I was in the same situation again, if I had to do it all over knowing what was going to happen to me at Immigration - I don't know what I'd do. My heart would say to take the chance and do it, but my head would say that being deported is too big a risk. I'd like to think that David & I, over a few more years, could have come to a decision to get married without having lived together, but I don't think that's the reality - so I had to do what I had to do - which is what you're doing. So I don't know.
And the thing you have to remember is that they DON'T need a good reason to send you back. They didn't have a good reason to send peedal back, she wasn't doing anything wrong, and they did. They didn't have a good reason to detain me for so long and harass me & David so much, but they did. Immigration has it's own rules, and all they have to do is *suspect* you'll be hedging the rules or doing something you shouldn't, and you're on the next place out. I felt safe resting behind the "I'm not doing anything wrong!" theory - but they didn't give a toss. You don't have to be doing something wrong.
I'm not trying to be a big scary jerk here, I'm really really not - and so I'm just going to shut up on the whole matter because hopefully, for you, things will turn out brilliantly and they'll just let you through no problems. I really hope that's what happens for you. I really do wish you the best of luck - and totally sympathize with your situation.