I've arrived at this conclusion by thinking through a number of biblical principles.
One of our bedrock governing principles in biblical dating — and in how we treat our brothers and sisters in Christ generally — is not to "defraud" our single brothers and sisters by implying a greater level of commitment between us and them than actually exists (see 1 Thessalonians 4:6).
Scripture calls Christians to "flee" from sexual immorality (1 Corinthians ), not to "see how difficult we can make the temptation and still prevail" or to "see how close to the line we can get without sinning." In my view, Scripture teaches clearly that there is to be romantic physical intimacy outside of marriage.
No reasonable person would argue that physical temptation does not increase — a lot — the longer two people date who are attracted to each other and who grow to love each other.
Over time, maybe you take some of the same classes, live near one another, etc.