In answer to your question, what I meant by physical attraction being sometimes there and sometimes not:
Well, I don't mean that I am not always attracted to my husband in some sense. But I'm not always thinking about it, and I'm certainly not always "in the mood," so to speak. (This is all the more true now that I'm pregnant. Keep in mind that marriage has a lot of phases, of which pregnancy is one, in which sex ends up being downplayed. When you're dating, you think about it all the time and tend to imagine that married people do too. That's not necessarily so -- which is part of the reason why physical attraction is not much of a foundation. After you get married, there will be all kinds of times when you're thrown back on your non-physical relationship, which hopefully you had plenty of time to develop.)
What I mean is that some moments, I look at my husband and my heart skips a beat and I think, "Wow, I am so attracted to him!" At other moments (and this was true even when we were dating) I don't tend to notice the physical aspect at all, because I'm absorbed in the conversation or activity we're involved in.
I don't know if men tend to feel the same way ... but in my case, physical attraction is kind of a now-you-see-it-now-you-don't kind of thing. That's why it's not the most important part of love, by any means.
I fell in love with my husband by stages. First there was the friendship stage, where I developed a really good intellectual relationship with him and had a lot of fun with him. Then there was the emotional stage -- fallen into headlong when he looked at me a certain way, used a certain tone, etc. Then the physical part came last. I was watching him playing soccer and suddenly I realized how incredibly attractive I found him.
To know if you're in love (particularly in the sense of discerning marriage), I'd ask the following questions:
Do I like this person? Are they a good friend?
Do we get along well? Do we complement each other and understand each other well?
Do I look up to, respect, and admire this person? One definition of love is an attraction to the good in the other person. Do I see good in this person?
Do I feel happy when we're together, in general? (That doesn't mean at every single moment, but in general, is this a person I enjoy being with?)
Do I trust this person? Do I feel like I can tell them anything?
Am I physically attracted to this person, at least a little bit? Is there enough chemistry that I really notice that they're the opposite sex, or do I feel completely like we're brother and sister? (I say a little bit physically attracted, because physical attraction is something that grows a whole lot once it gets going ... so you really only need a bit to start.)
And of course, Do I have the most important things in common with them? Here I mean faith, the importance you give your faith, your views on children, roles within marriage, money, and so forth. You should definitely discuss these things with anyone you're dating.
Well, that's just a little advice from someone who is definitely no expert. But I hope it helps all the same.