Just how many bisexuals are there out there? Some people I've talked to doubt that there are many, and suspect that there are many more homosexuals than bisexuals. Here's some data on this, though, from Laumann et al., The Social Organization of Sexuality 311 (1994). Of course, even though this was a big and seemingly well-conducted survey (3000+ respondents), it got only a small number of responses from homosexuals and bisexuals, so the numbers might not be that robust; and of course there's no guarantee that the respondents are reporting accurately. Still, it's the best that I've seen:
Sexual attractionAmong menAmong women
Only opposite gender93.8%95.6%
Mostly opposite gender2.6%2.7%
Both genders0.6%0.8%
Mostly same gender0.7%0.6%
Only same gender2.4%0.3%

If you look at the reported sexual practices (not just attraction) of people who have had some same-sex partners in particular time frames, here's what you see (numbers rounded off):
Time frame in which the person has had some same-sex partnersFraction of male respondents (the ones who had some same-sex partners) who had partners of both sexesFraction of female respondents (the ones who had some same-sex partners) who had partners of both sexes
In the last year25%25%
In the last 5 years50%60%
Since age 1880%90%
(As best I can tell, the time frame in the numerator is the same as in the denominator -- the 50% number, for instance, means that 50% of the men who have had a same-sex partner in the last 5 years have also had an opposite-sex partner in the last 5 years.)

     So there do seem to be quite a few bisexuals out there, though how many depends on how you define bisexuality. This suggests that the law (for instance, law limiting marriage only to male-female marriage) might indeed affect people's choice to enter heterosexual relationships as opposed to homosexual ones, whether by direct incentives (e.g., financial benefits) or by affecting social mores. If we thought everyone was either firmly heterosexual or homosexual, such effects would be extremely unlikely. But if quite a few people can go either way, then they might be swayed by legal or social pressures.

     Naturally, one might still think that the law shouldn't try to sway people in this respect, or that even if it's good to push some people into heterosexuality, the costs of any such push outweigh the benefits. One might also be generally skeptical that the law will have that much of an effect even on bisexuals. (As I've mentioned before, I tentatively support allowing same-sex marriage.) But the presence of bisexuals suggests that it's not crazy to think that the law might indeed have some effect on how many people end up in heterosexual relationships as opposed to homosexual ones.